I know it's too late now but here's my insight on it.
leos are from the middle east, very hot there, so their warm side needs to be around 90F, cooler side seventies to low eighties. as they are cold blooded, this gradient allows for proper thermoregulation. I recommend ceramic heat emitters as they do provide proper gradients. leopard geckos take in more heat from their bellies so under tank heaters can be used, but this does not create a gradient and thus does not allow for very efficient thermoregulation. under tank heaters also cannot be used with deep substrates.
20 long is fine for an adult, for the sake of space, but you can go larger, if you clutter the thing with hides and get a larger heater/multiple water sources to compensate. they are not as easy as people make it out to be.
the main thing with large enclosures is it's harder for the gecko to find essential things like heat food and water, and it's a bit harder to clean. so if you were to go big, I'd get an adequately size ceramic heat emitter hooked up to a thermostat on whatever side you want to be the warm side for the thing.
As for setup itself- if you want a bioactive (naturalistic) setup, do a substrate mix with 10 parts- 4 organic topsoil (any soil without the little white things or chemical fertilizers), 2 play sand, 2 cocoa fiber, then 1 part sphagnum moss and 1 part dried almond leaves. with this substrate mix you can plant succulents. succulents such as haworthia or echeveria (or other similar succulents) are safe for the gecko. this substrate also allows the gecko to burrow, though you will need to feed it by hand. as for hides and other hardscape, you can use segments of cork bark and ghostwood/grape vine for hard decor. cork bark can be half buried to allow the gecko to make its own burrow (which it will)
hides should be added on both sides of the enclosure, to again provide proper thermoregulation. one hide on the warm side can have an absorbent media like sphagnum moss in it, to make the inside of that hide humid to aid in shedding. other than that hide, the rest of the tank should be very arid, as too much humidity for too long can cause respiratory issues.
leopard geckos only poop in one spot, usually a corner, and that spot is usually the spot they spend the least amount of time in. so if you want to make cleaning slightly more efficient, don't decorate one corner of the tank that you want to designate as its 'bathroom', as to give the gecko no reason to be in that corner other than to use the bathroom.
do not use straight sand, no matter how fine, nor gravel, wood chips, calcium sand, or bark as substrate. calcium sand is probably the worst of it, because usually dyes are added to it which ends up staining the underside of the gecko, and geckos are attracted to calcium like deer are attracted to salt, so they will lick it (and in the process pick up sand) and end up becoming impacted, as they cannot pass the heavy amounts of sand.
as for water, on the setup I suggested before, just have a bowl about an inch high but only fill it up half an inch, just for the sake of keeping substrate out easier. smaller geckos especially have a hard time getting out of even inch deep water.
now if you just want a safe setup you can mess around with excavator clay and fake decor, again with a ceramic heat emitter. sand can be mixed with excavator clay to give a sort of concrete effect, as in it sets to a harder, more solid form that the gecko will not be able to dig in.
either that or you can go bare minimum, and have paper towels as a substrate (easiest imo) and have any decor you want as long as it's safe for reptiles. I do not suggest getting decor from outside, as this may contain parasites or chemicals not safe for the gecko.
as for cleaning decor- hot water. or veterinary disinfectants for non porous things. never use any sort of chemical to clean wood or plastic. don't use ceramic decor either, as it is porous and will harbor bacteria, and thus cause problems later on (the whole harboring bacteria thing is why we aquarists use ceramic balls as filter media) either way you shouldn't need to clean decor very often unless it got pooped on or something. they do occasionally poop where they aren't supposed to.
while it is a controversial subject, I suggest leaving a bowl of calcium, with d3. and not dusting feeder insects with such, other than vitamins. my reasoning: if you are forcing the gecko to eat an amount of calcium it does not need, it can (possibly) end up having a calcium overdose, which can lead to bone fractures. calcium and d3 are still required, as providing calcium as a supplement prevents things like metabolic bone disease, and d3 is just a vitamin that allows for the processing of calcium in the body. like I said before they tend to lick calcium, as they understand that they need it. I believe it is better to leave it to the gecko to dose itself in terms of calcium intake, as it will only take in what it needs. if it already has an apparent calcium deficiency however, then is when you should dust feeders to make sure it is getting a good amount of calcium. and again, leaving calcium with d3 allows it to absorb the calcium it takes in.
now as for feeding, adult geckos should be fed two feeders per inch of gecko, once or twice a week. length of the feeder should be the width of the gecko's head. that's the rule of thumb I go by.
babies should be fed once everyday, juveniles once every other day. same rules for feeder count and size.
never feed the gecko insects you find outside, or around the house, as again, they may carry parasites. if feeder insects escape, either kill them or if you can, feed them to your fish. that's what I do.
in terms of looking for ailments in the gecko, the main thing to be worried about is runny stools. runny stools in geckos can be signs of alot of harmless things, like sudden diet or supplement changes, too much water intake, or too much activity, but then especially if they're consistent and/or discolored, attention should be brought to it immediately. you should also check on the gecko's toes, as when geckos shed, if it isn't humid enough for the skin to come off smoothly, it can get stuck around the toes, and if left unchecked, the stuck shed can build up and cut off circulation to the toes, eventually leading to the loss of said toe. to remedy this in the event of it happening, you can dip the gecko in warm water (barely up to its legs) for a few minutes, then brush the stuck shed off of its toes gently. same process for any other stuck shed. shed should only be deemed "stuck" if it is there for at least a day after the gecko sheds.
as for handling, scoop from underneath, unless the gecko is very accustomed to handling or is escaping and you drastically need to get a hold of it. reaching from above stresses them out as it makes them seem like they're being picked up by a predator. never grab by the tail, as their tail is their fat storage unit, which, in adults, allows them to go months without food. feeding adults once or twice a week keeps them at a good weight, but too frequent/high fat feedings can lead to obesity. the fat in their tail keeps them from needing to eat frequently as well. hence why the tail is essential. grabbing the tail may trigger their defense mechanism, which involves them rapidly detaching the tail by a set of muscles wired to disconnect in the event of an attack. the detached tail also wiggles for a few minutes, which in nature would act as a distraction. they grow their tail back after a few months, but it doesn't look the same, as the normal tail is relatively complex and contains vertebrae, while the regenerated tail grows back with cartilage and does not grow the scales or tubercles back. it just grows back to serve its purpose as a balancer and a fat storage unit, so it'll just look like a raddish or a bulb with some of the coloration from the old tail.
oh and the main thing I tell people- always wash your hands before and after handling your gecko, or anything inside the enclosure that it would touch.
hope all of this helped, while it's not everything you need to know, it's a good amount of information I deem essential after years of keeping leopard geckos.
