Yeah I'd consider at least 60w maybe more because it will take a lot of energy to reach the floor. Imo enlosure size won't stress the animal out providing you give it enough hides and cover so it won't feel out in the open but may effect its ability to catch prey before supplements wear off. So one way is to provide lots of cover and then tong feed or preferably raise in a smaller tank so it can catch its prey itself, if you're frog is really small. If you can establish the right temp gradient in the tank you have it would be ideal. If you can't, I'd highly recommend a large plastic tank and then putting the heat mat underneath, connecting it to a thermostat so the floor temperature (underneath the subtrate) measures around high 70s to low 80s, this will not burn the frog but will provide it enough heat. Many people will go against this but fact is at the beginning most horned frogs were probably raised like this and fared well, it is certainly better than having the mat on the side and not providing the right temps, which will cause many health problems. If you need any guidance, my bible with horned frogs is chacoan horned frogs by Philippe de Vosjoli