size dosn't really matter with motorbikes , im 6ft3 and drvie an R6 aswell as my friend who is literrely 5 ft3.
most dirt bike come sized by the litre (cc) most 125cc are powerfull bikes (try to get a bike with powerbands) great fun!
Now heres a thing or two i think you should know , 2 stroke are ussualy faster but they arnt put together as good , 4 stroke are very independant.this is because most new bike are 4 strokes (new technoligy etc etc) 2strokes you'll will have to keep re-newing part especially the carb etcetc
good luck finding your bike , remember Dont Drive it , RYDE IT
I started on a 125cc 4 stroker, I remember going to pick up the bike and getting on it, I had only been on a mini bike prior to this, I reved it up popped the clutch and rode a wheelie across the yard. What a feeling, I am sure when I came back I was white as a ghost, I loved that bike.