I have two snake rooms and I have venomous and non in both rooms. Both rooms are 11' x 13 1/2'. One room has adult venomous and 3 big Pituophis. The other room has babies, vens and non, adult Corns, Kings, Milks, Juv. Pituophis and 2.1 adult King Browns.
I'm about to finish the inside of a steel building that I will be moving my adult venomous and non-venomons into. That room will be 20' x 25'. I no longer keep Mambas, but I do keep mainly Cobras, including Kings, and PNG/Oz Elapids.
The more room the better, but if you use trap/shift boxes for the very fast stuff, Eastern Browns, Taipans, King Browns, Mambas, Papuan Blacksnakes, etc, it makes things much safer and easier.
RC