At your size I would not worry. Years
ago I bought a 18 month old Arabian. At 5 years he matured at 13. 3 hands.
After doing one limited distance, I decided he was too small for me ( I'm a
middleweight). I gave him to my daughter, who started doing
endurance on him. My daughter was about 5ft 6in and 100 pounds at the time. In
the following 12 or so years they finished over 2000 miles including 7 one day
100s. The got several top tens and finished Tevis.
Do not give up on a small horse, Also they
can grow quite a bit between 3 and 5. Ted
Goppert