I make bias binding and I use the french double fold technique. I'm typically working on bed sized projects and I figure it's my last chance to choose a fabric so I make my bindings a bit wider than average, I typically cut at 3". There is an extra long ruler out there that is 2.5", that's a help and sometimes I use that, but mostly 3", and I've gone bigger now and then.
I do not trim my quilt until after I put the binding on. I cut 1/2" from the seam line, that gives me a full binding, and that seam is usually easy for me to see.