What was it about the 52 cm bike that felt so much better to you? Can they let you try it with the stem they're thinking about?

I'm 5'3" and I'm riding a 50 cm Cannondale WSD bike that's possibly marginally too big for me. But it all depends on your body proportions. Every manufacturer has their own sizing conventions too now, with the sloping top tubes - it's not like the old days when it was either center-to-center or center-to-top and the frame size was the actual measurement of the seat tube. With a horizontal top tube, my old custom frame was 47 cm c-to-c.

When you say your "knees were too bent" on the 50 cm bike, do you just mean seat height, or something else? Was the saddle raised as high as the seatpost would go, and you still couldn't straighten your legs on the pedal stroke?