28qts should be fine for adult males. 28qts have the same floor space as 32qts, but they're just a couple inches shorter. For large females I'd go 41qt. 6qt is fine for hatchlings, then you'd need to move up to 16qt for juvies, then to 28qt/32qt and on to 41qt if they get really big.

When you're trying to feed frozen, is the food warmed up a bit? I've also had good success taking a little bit of bedding from my pet rats and putting it in a corner of a snakes tub, and then rolling the thawed food item in a little soiled rat bedding as well, to help give it a strong scent. Some snakes just hate f/t