According to the rules of the World Chess Federation, the size of a square should be twice the diameter of a pawn's base. It is recommended that a side of the square should measure 5 to 6.5 cm (roughly 2-2.5 in).
If you really want to get fancy, make the "squares" slightly rectangular - a bit longer than they are wide. That creates the optical illusion of them being square when you sit at the end.
Regards,
Eric