I'd say after 6qt, you could go to 12qt or 15qt.
Check out C-Serpents racks fo size examples- http://cserpents.com/C_Serpents/RACK_SYSTEMS.html
Hatchlings in this
http://cserpents.com/C_Serpents/Hatchling.html
Sub adults/adults in this (I have a 1100+g 0.1 Pied and several ~450-1100g snakes in these tubs)
http://cserpents.com/C_Serpents/V-18.html
Breeders in this (I have my 1500g male Pied and my 1300g 0.1 Spider in here. My 1100g 0.1 Pied will be moving in this week).
http://cserpents.com/C_Serpents/V-70.html
I dont use hides in any rack tubs except for one. I have a 1.0 Bumblebee hatchling that will just not stop wandering his tub unless I put a hide in there. He seems very stressed without a hide.