Yeah with a lanky form like yours, you might feel a little cramped on a 750. You could get extended foot controls to help fit. Then again, in the used range, the bigger bikes are often the same price as the mid-range ones.
As others have said, don't buy anything you have any doubts about. If the bike don't fit, you're better off to keep looking.
Make sure you hear it running, and test drive if possible. You'll soon notice the difference between a smooth purr and an iffy whine or grind. The bike in good condition will just sound quieter, cleaner, and "right".
Be aware that wheels often need changed on used bikes. And have it serviced ASAP by a dealer or a reputable repair shop.