The rule of thumb I was taught, is that if a dime can fit through it, a mouse can fit through it. Mice are an annoyingly good incentive to keep your dwelling well caulked.
A rule I came up with myself (due to living in an 18th or 19th century factory converted to a house, which was well infested with mice when I bought it) is that anything a mouse can fit under or into should also be big enough for a cat. This rule determines things like the height of bookcase legs, and so forth.
