When choosing the size of studio monitor, you have to consider the room you are mixing in. The size is based around the diameter of the woofer speaker which commonly varies from 4 to 8 inches.
If you are mixing in your bedroom then 4 inches is plenty and even if you’ve converted your loft or garage into a studio then 5-6 inches is easily enough. Any more than that and you’re not going to benefit from the amount of air they can move. For larger rooms and well-insulated spaces, the 7-8 inch speakers will do the job nicely.