I'm 6ft, 185 and have ridden a 96 yz125. good bike but you do have to work it.
if you have good clutch and throttle control, most 250's would work fine. a 125 is lighter and feels lighter, but a 250 would give growing room. seat to peg to handle bar height is a factor for us taller people. I have friends who have started out on 250's. with proper instruction, they did fine. watch the bikes with sharp power curves, they develop power fast and can be a handfull. different pipes can help this.
just remember, good clutch and throttle control is a must and do NOT drop your shoulder as this causes you to grap too much throttle. have respect for the power and you will be ok.
I do not race, just play hard and am partial to YZ's. Some KX's and CR's have sharp power curves. YZ's tend to have more low end, but that differs from year to year. It would help if you had friends w/different bikes that you could try out. Just remember to pay for what you break. good luck.
93 WR 250