I mainly used 1 1/2" but I guess 2 1/2 could be used also.and I do the same as Mkotch by sewing them as leaders enders and I have used them différently and a lot of time, for the back of a quilt.
I just make sure I sew a light and a dark fabric together
I am currently sewing 2" and 2 1/2" four patches
I see a "scrappy jewel box quilt" in my future