The tank has a cut out where your leg “should” go. It helps you hug the tank better with your knees. For me, the top edge of the cut out is right in the middle of my leg, so squeexing the tank is uncomfortable. You may actually find that if your lower leg is long enough it’ll raise your leg above that edge, and you can grip the tank comfortably above it. I dunno, you’ll have to try.
I do know that I’ve seen some fairly tall guys (6’4″ish) on 250Rs and they didn’t look like clowns. Really, very few people will ever actually notice how you look on the bike, or its size in relation to you. Frankly, if you never point it out, most people won’t think twice about it.
As for the power of the Ninja 500. I haven’t ridden one. I can’t comment on its power delivery or throttle control. I’ve ridden an Interceptor 500, which made more power, but was still forgiving with the throttle control. It had other issues, the biggest one from a learner’s perspective being the confidence it instilled. My ZZR-250 was happy doing slightly more than the speed limit. The Interceptor wasn’t happy until it was laughing in the face of the law… The ZZR-250 was certainly a better bike for the longevity of my license.
If you are concerned about the power, consider a Suzuki GS500 as well. It makes slightly less power than the Ninja 500.
The new ones look pretty hot in full fairing (imo). I haven’t ridden a GS500, but I’ve spent a lot of time on its ancestor, the GS450 (which made abotu 10 less horsepower). The 450 is quicker than it looks, but incredibly easy to manage the power on. I *think* the GS500 also has more leg room than the Ninja 500- but again, I haven’t sat on one.
It really should be on your list of bikes to sit on if the 250R doesn’t work out for you.