Fire eels can reach 3 feet or so, mine currently is nearing the 16 inch mark. I would avoid getting the red and rainbow sharks (at least not more than one) they are very territorial and won't tollerate their own kind unless you have a very large aquarium with enough places to claim territory. Bala sharks are a great fish, but they can reach more than a foot long and are a schooling fish, so you can imagine that these would require a HUGE tank. The ghost knife will reach about 20 inches, but it is a rigid 20" so they do require a fairly decent size aquarium. Ghost knife and clown loaches seem to grow quite slowly. I unfortunately lost my ghost at about 8 inches, but I will try again down the road.
Yes the fish I mentioned do get quite sizeable, but a decent setup could meet their needs for a few years. My fire eel started out quite small and has reached its 16 inches in the course of 2 years, I imagine he will begin to slow down at the 2 foot mark.
If you have a smaller aquarium and want some nice shark type fish, I would recommend Denison barbs (red line sharks). They max at the 6 inch mark, have beautiful reds and yellows, and are fairly non aggressive.
Silver dollars should be fine with any non aggressive or semi aggressive tank (fish eat anything that fits I'm their mouth) but from what I hear, silver dollars are waste machines and have a fairly large bio load.
If you have the smaller tank and have a peacock eel, they don't get quite as big and you can actually have several in a tank. The larger spine eel varieties like fire and tie track are usually one per aquarium (though I have seen multi in particularly large 400 plus gallon setups).
Size is a consideration when stocking your tank, but if you plan it right you can house even a fire eel in a 125 gallon for several years with no issues. It all depends on what you want in the hobby. Monsterfishkeepers.Com is a site for addicts, and if you get the aquarium itch, you will start pushing your limits and move up to bigger better tanks. If you are a beginner, stick around and visit the different threads, each section has their experts and good advice. Knowledge really is power when it comes to caring for any animals, and this place is full of the experiences of hundreds of people. Eventually you will find that dream fish and a setup to build around it, until then just enjoy the aquarium hobby.