How big is too big of a tank for an 18 inch corn snake?

i just got my tank set up for my corn snake, i moved it from a 5 gallon reptile tank to a 20 gallon fish tank, he seems to be taking to it nicely, i made a hut out of rocks, nd i made sure its sucure so it wont fall on it, i have alot of rocks in the tank also, a few plastic plants around the water dish, i log, about 1 ft tallor more, with holes drilled in it 4 the snake to crawl through, nd a small deer skull(yes its real) in the middle of the tank with a few rocks behind it,(im watching my snake right now(its name is waffles, idk if its a boy or a girl so i figured thts a good name lol) so anyways im watching "waffles" crawl around the tank, in the 5 gla he didnt move, but now hes going everywhere, jst exploring, the tank dnt look to big 4 it with all of the rocks and things i put in, i heard tht sometimes if the area is too big tht the snake will get stressed out nd die, idk if thts true, he has plenty of hdeing spots too, so, with everything in the tank as explained, the tank shouldnt be too big right?? im pretty sure its not, but, i paid $50 for this snake nd i dont want waffles to die on me, i really like him, evn though almost everybody in my famiily is afriad of it, cept for my sister dad, nd 2 brothers, nd is there any things tht corn snakes maybe like to do, like climb or somthing, cuz idk , nd i can go out nd find plenty of stix to put in his cage, i wash em off with really hot water so there aint nthing to kill my snake on the stixany extra info would be great on cornsnakes, i have a care sheet printed out so im good there, ive only had waffles for a week so still earning stuff bout cornsnakes......thnx!! :)