Above 10" your viewing improvement is very much determined by Seeing conditions. I have had Dobs up to 16", but they get pretty hard to handle at that size. As suggested you could get out to a star party and check out a few different sizes, including the sheer mass of the scopes. Consider if you have to fit it in a car or where you will store it. Good luck and beware of apeture fever ( you quickly get used to any improvement in apeture and seek more, and more,...).
