Help with picking my first real dirtbike!?

i am 5'7 140lbs i have had a few pit bikes but not a real bike. and i have saved up spme money to get a real bike what would be good for me? kx100? or a 125? i would really like to get a 4 stroke because i will be riding alot of trails. do you think it would be crazy for a 250 4 stoke for my first bike?