I myself use pine cages about 500x450x450 with 12mm wire mesh front and top. Went away from tubs because the heat and humidity up here (SEQ) made the tubs 'sweatty' on the inside (beads of water inside the tub, dripping down), even with a mesh top. You cant drill/melt any holes lower down as the rats will chew out of the tub. The lack of ventilation also promotes mycoplasma (urine smell trapped at bottom of tub) I have seen household bathtubs used to house large number of rats (mesh top) and I have seen rats in 'drawer' type setups, not much bigger than 300x250 x190 with a mum rat and babies.
The rats neccessities are fresh water, adequate food, adequate ventilation and room to move. The last thing, room to move is up to your moral standard. If you look at your rats in a little tub and think "hmmmm thats a bit small" then get bigger tubs. If you look at your rats in a small tub and think "who cares" then that is that.
I am like you, I molly coddle my rats and look after them really well, almost like pets.... but in the end they usually end up a snakes dinner. My motto is "they might be here for a short time, but it will be a nice time"