What cutting diameter is the most popular? If I were to go down from a 1 1/4 it only make since that it would make the arrow fly truer. What is everyone using and why?

1 1/16 to 1 1/8 for fixed blades.Ive used smaller but wasnt impressed with the blood trails. You couldconsider an expandable also. Even with a perfectly tuned bow any flaws in your formwill show up dramaticaly morewhen you are shooting fixed blade broadheads.The newer style super short heads and expandablesare more forgiving to shoot because of the reduced blade surface but still carry good cutting diameter.

In a fixed head, my preference is the Slick Trick standard 1'' four blade.
With the 1'' four blade you get 2'' total cut.
A 1 3/16'' three blade makes a little bigger hole, but actually doesn't cut more tissue: 1.78'' total.
The 1'' Slick Tricks fly great from a paper tuned bow.
Either will work, but I prefer the 4 blade.

Haven't used anything over 1 1/4" for about 2 decades, but most recently (10 years or so) I've been using heads that are 1 1/16" to 1 1/8". Yes, they are easier to tune because of lesss wind planing, and they are big enough to fo the job.