If you've got the clutter, nothing is technically *too big*. As Alicia described, warm hide cool hide, then as much stuff as you can fit in all around and between. Then even a baby would eventually feel comfortable. Not that I'd ever recommend a baby in a 40 gallon breeder, just mentioning that with enough clutter and proper hides it could be done. It's easy to make it homey enough for a yearling though.