bringing up a semi-old thread here.
Here at LCMNH I use an array of tank sizes for our herps. I have as small as 2 gallon tanks up to custom habitat of 8x8 ft. But one habitat I have fits this post, our western hognose tank which is a 20 gallon long. Ambient temp in our snake room is around 73-75°F. I have 2 lights on the hognose, a reptisun 2.0 fluorescent and a 40 watt reveal bulb. Reading this thread got me to go check basking surface temps. Under the 40watt bulb temps are reading 94.5°F directly under and 91.5°F 3-4 inches away from center bulb illumination. Temps were taken with a RayTech IR temp gun. Even with a room temp in the 60s a 100 or 75 watt bulb will most likely be way too hot. Case in point, our collared lizard/western marbled whiptail/twin-spotted desert spiny tank has 2 basking bulbs 12inches away from concrete faux-rock work. Bulbs used to heat the basking area are 1 Active UV Heat 100watt flood and 1 GE reaveal 100watt. Surface temps at direct bulb light concentration read at 135°F surface temp (rock surface temps outside here during the summer reach 150+°F). Surface temps between the 2 bulbs' concentration of light read around 123-128°F.
another tank we have is our madrean alligator lizard/great plains skink/new mexico whiptail tank. We have 1 basking and 2 fluorescent bulbs on the tank. The basking bulb is a GE reveal 60watt placed 8 inches away from a rock surface. Rock surface temps were measured again with a RayTech IR temp gun back on Wednesday and read 106°F.
Now to take this away from work, I have at home a 10 gallon tank housing 1 of 15 leopard geckos. I use a 40 watt bulb to heat the tank. My herp room typically keeps a minimum temp os 68°F. The basking location within this 10 gallon tank, with a 40 watt bulb, places temps ranging from 85-91°F depending on the temp of the room.
So from a professional and personal stand point of experience, a 20 gallon long (roughly same height as a 10 gallon) should be fine with a 40 or 60 watt bulb for a basking area. Anything larger and you face keeping the animal too hot resulting in either it hiding on the cool end ALL/better part of most of the time and/or not eating due to too much heat-related stress. And if it does eat it may regurgitate from being too hot. Regurgitated pinkie/fussy/adult mouse cooking in a hot tank = decomposition perfume
