Base board moulding is usually 3.5 to 4.5 inches. The 2.5 is used for windows and doors.

Crown moulding runs from 4 to 6 inches.

Due a test in your house. Go to your local HD or Lowe's and pick up small sections of base board and crown moulding. Usually in the discard bin.

Do a mock up to see what works for you.

Enjoy the journey.

eal51 in western CT

The size of the molding is also contingent on the house style, room size and it's age.
I have a 1949 colonial with 8 " base molding with a shoe molding at the bottom. Ceiling only has a picture hanging molding(1 1/2"), no crown. New kitchen addition will have a 4 5/8" maple crown with a rope insert.
You want the crown molding to be proportional to the room.
Ron