I use 4 sizes. 6qt for hatchling racks. I have a few racks that fit 2 12qt or 1 28qt per shelf for flexibility of moving smaller babies up without having to build a new rack. Then I have my adult rack for female breeders only with 41qt.

6qt - hatchlings to 200g
12qt - 200g to 600g
32qt - 600g to 2000g (or adult males)
41qt - 2000g + females

I've only had 1 male ever in a 41qt and he was over 3000g. Biggest male I ever had. Also if I move them up I make sure there is more in their tubs. For example an 1800g female in a 41qt tub will have 2 hides and a large water bowl. If I have a 3500g female in a 41qt tub, I just give her a water bowl with no hides.