That seems large to me, but your overall height isn't the only consideration, it's inseam as well. Have you taken the bike for a test ride? If there are side streets around the shop, that is ideal, not just around the parking lot.
I'm about 5'7" and feel more comfortable on a 17.5" hybrid.
It depends on what you are going to use it for. If you are going to jumping off the curb every few feet as you go then the 20 in is the one you want. It's more rugged. You will notice that there are NO 26in jumping bikes out there because the wheel will self destruct after a jump or two or three.
If you are going to use it to ride from point A to point B then the 26 is the way to go.
I am 5'10" and over 200 lbs and feel silly riding a 20 in. I see the youngsters do it and they peddle like crazy to keep up. Of course, when you are that young then you need to burn up all that energy somehow. But it sounds like you need either a 24 or a 26 in for regular street driving.
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