My advice would be start on a 20" and then switch to a 24" when you can ride comfortably. I am 185 cm (about 6’ 1") and I started on a 24" because I found pedalling smoother on this compared with a 20". However, after some time I realised that mounting was easier on the 20" - and that this was more important when I started to ride with only limited support for mounting.
This was my training area for the first period (in April this year) - made with a ladder and some gaffa:
After about two weeks of daily practice, I switched to a parking lot with support only for mounting. On the first day I made some 30 m rides and within a week I could ride 150 m. 
