This depends on a few things: age, weight, height, balance and skill. Below are some guidelines to consider.
30" Wheel Cut Board
Usually smaller kids will ride these. easy to carry, and not taller then you! We see more young kids ride this board. Buy if you have good balance and weigh under 120lbs.
40" Wheel Cut Board
Great all around size. Looks and feels like a long board. Buy if you want to carve and are good at riding. Good for any height or weight. A little experience helps with this board.
40" Platypus / Platypus Drop Through
Good for all ages, all sizes, all experience. The go to board for anyone - make it the ultimate board with the drop through. It will be lower to the ground, offer better balance, and made for speed.
Good for all ages, but it is heavy and super long. Young kids need to make sure they know what they are getting - it weighs 12lbs. It's heavy.