I was in the same situation as you as I only started trials this year and about your height. I started on a 20 but always found I was too big for it and was constantly trying to make it fit me. Then I switched to a 26 and was much more comfortable. I would say it depends on your budget, if you want to start cheap get a 20 and fit a 180-35 stem as this seems to be how taller riders set up theirs.