We wouldn't recommend buying any bike without sitting on it first. I can't imagine you'll be doing a lot of mileage on a 125, so you shouldn't be too uncomfortable anyway - but I'd recommend you have a look at a Honda XL125V Varadero. It's a great 125 for a tall rider but it's getting on a bit now - and isn't as stylish as a CB125R - but as long as you check the history then you should be able to find a good used model for reasonable money.