I can understand for breeders to have 6x3 to save on space as they can do find in them and thrive, ask Sam Bearden.
But if you have the space and you only have 1-2 giving it some extra room by having a 8x3 isn't bad either. I am going this route only b/c I have 1 burm and feel like giving him the extra space (and b/c it will fit nicely on my 8x3x3 tegu cage I am building in June haha)