Mine are all 3/8". It works well for me. I also notice that I never get much debris build up in the winter as I used to have with 3/4". The bees keep it cleaned out.
In the summer, I slide a super above the hive bodies back about 3/8" for an additional entrance when needed.
In the winter, I fasten a reducing block to the bottom so that the bottom entrance is about 3/4" wide, and keep a 3/4" round hole near the top of the hive open.