The discovery of the bee space is said to be the foundation of modern beekeeping.
For a long time I thought it was a very simple matter and didn't give it much thought, but as I've been attempting to build my own hives I've discovered there's a range given for exactly what the bee space is (6mm - 9mm or 1/4"-3/8").
I've also learned I need to account for wood shrinkage and different parts of the hive need different kinds of space (ex. between boxes or between combs).
In the linked article Dave Cushman's provides a great overview of important space dimensions used in different pieces of hive equipment.