Hell, I used to ride a 20" six speed. If you're comfortable, I don't believe it's too small.
However, it's easier to ride taller bikes long distances without fatigue.
If you're any good with welding, you may have the room to upgrade your rim size without getting a whole new bike... you could get 26" rims, they might fit.
In fact, fronts probably will go right on, rear might need the brake bracket moved up.
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