Its really all about preference to be honest.
I can tell you for a fact that my first snake was a milk snake that I had in a 20 long. Its hard to maintain humidity and cool side temps can get a little hard at times too.
How I just got my first corn about a month or so ago and I have her (4 foot adult) in a 105 qt sterilite tub I bought from WalMart. 105 qt is a bit excessive for most of the people here who use tubs, but this is a pet snake and not a breeder. If your interested I can detail a bit of how I made the tub more show worthy and less like, well, a plastic tub
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I drilled holes for zip ties on one side of the lid and tub to give it the hinged lid effect. This also secures that side of the tub very VERY well. With two latches that came with the tub I now have 3 sides that are impervious (or as much as can be expected) to escapes. On the second long side, opposite the zip ties, I use two binder clips. The end result is a tub that I am extremely happy with!
Heating follows the same procedure as any other tub or tank. The bottom should be 1/3rd to 1/4th covered with a UTH.
As far as the tub melting from the heat, do not be concerned. Although UTH's can and do reach temps of 110 or 120 degrees, its rare that you hear about someone in Arizona having plastic storage containers in their garage just suddenly melting/bursting into flames. It can really be avoided if you do two things that are mandatory in my book. Proper ventilation underneath and a thermostat.
Feel free to PM me if your interested in more on how I setup a tub for display purposes!