My khuks seem to fall into categories:
14" to 15.9" = The small fellows. Can do more than you'd expect, given time and skill, just take somewhat longer.
16.5" to 18" = The right size for most work. They have the leverage to do more work with less effort quicker. They don't seem hardly more heavy to pack than the littler size. The lighter ones are very quick, and the heavier ones aren't too heavy - they just have the ability - well, if they were baseball bats they'd put the ball over the bleachers and out of the field.
20" to 22" = The light ones can just about be cracked like a whip. You can literally HEAR them cut the air. The choppers in this range if you've the strength to use them have awesome power. If they were rifles, you'd carry them for Rhino, Cape Buffalo, and Elephant.
Well, that's my perspective, at least. ( I'll talk about the 12"ers another time. )