like 2000 grams and up, I just had my one female in a 34qt. I just got her the 41 cuz i figured she could be a little more comfortable. But again I don't know what most people do, i have heard of people using 41's so I thought it would be a good idea, but I don't wanna waste space like you said. I wanna utilize as much of the room as possible being that it isn't going to be huge.