I'm your height, about 13 stone, and started trials about a year ago in my mid 50's.
I wanted to improve my bike skills as I do a lot of Enduro style MTBing, and fatbiking in the winter (4 months of snow here in Ottawa, Canada.) The trials skills have helped a lot!
I started with an Inspired Flow Plus (24) street trials, then added an Echo MK5 (24) comp style. The Flow is more similar in feel to my MTB & fatbike, except smaller in all dimensions. The MK5 is much lighter, but feels so different from the MTB & fatbike. I've never tried 20" bike, but my guess is it will feel even more different. Personally, instead of the MK5 24" I probably should have bought a 26" pure. But I'm happy with my choices.
I'm going to try to learn more trials riding this summer, so I'm also going to try to get down close to 11 stone.