On full sized quilts I use 2 1/2 double folded, usually on the bias as it wears longer, but have used straight binding as well.
I sew lots of small quilts, table runners, wall hangings, etc. For these I like to use single folded binding cut 1 1/4". Before I sew the binding on, I firmly press the edge not sewn to the quilt 1/4" under and sew the unpressed edge to the quilt. Now I have the folded edge in place to either machine or hand stitch.