Inside length of at least 68 cm (1.5 × your cat). If you measure to the tail tip instead (the AAHA/AAFP way), the box ends up longer, so go bigger if you can.
A planner built on the one-per-cat-plus-one and 1.5× length rules from International Cat Care and AAHA/AAFP. It's a starting point; your cat's own preferences win.