The only thing I would worry about with a smaller frame is toe overlap where the front of your shoe contacts the front tyre because of the shorter wheel base. Then again I've had this issue with frames the 'right' size that have a compact geometry. As for comfort, best fit for longer ride...well make all the adjustments you can and go ride...and see how it feels. I think the OCR sizing is Small, 51cm, 5ft 2 – 5ft 6 and Medium, 54cm, 5ft 6 – 5ft 11. It still depends if you're short in the leg, long in the arm, normal in the upper body or any combination of the three
But if it feels comfortable then happy days.