A little trick I use to keep my sizing fairly regular is to visualize a marble (or sometimes one of the larger buckeye type marbles) as the diameter of the loopy bit in stippling.
I've hear other people who visualize puzzle pieces. IF you do a lot of jigsaw puzzles, you have a sense of the basic size and shapes, and can translate that to your stippling.
I actually find the act of stipple quilting to be very boring, but I think it is often a good choice of quilting style.
RST