I have 2 t8s (4x2x1) I personally would not want to go any smaller, but im sure others will say you can go less (if I remember correctly a 41qt is 36x18x6).
Im with you on not wanting a rack, the function of them is nice if your a breeder, but in my opinion tubs are to small for the snakes
Also for my t8s I have two 40 watt RHP's (1 per t8) on a herpstat 2 and I have had no issues, I LOVE the setup.