>>9888369I agonized over this for a little bit, and decided to go with 4x4 for 5 feet.
A 3x3 stud square is almost exactly perfect for a standard 1 inch grid that most games use, but would be so much more obnoxious to build around, and going with 4x4 as the standard gives me a lot more space to build monsters around.
Also doors are way easier this way, too.