Mine is a smaller one. 4x8' Seat about three.
It's really all up to you. Looking to have parties? Then maybe something 8x8' would be best. Just for a couple people? Then 4x8'. The more space you have, the more comforts you can have. Bad things about bigger shacks is that obviously they are more expensive, but also harder to heat and the weight is a PITA to move it around. Before you even start building, know exactly how you will move it.
Tell us what you want to do or take a look through the shanty topics and see what people have.