I see that Rose have changed their frame size calculator online and made it a lot simpler. It used to act like most online configurators and ask for multiple parameters, now it simply asks for inside leg only.
- If you enter 77-80cm inseam it tells you 53cm frame
- If you enter 81-83cm inseam it tells you 55cm frame
- If you enter 84cm inseam it tells you 57cm frame
So you can see that we are boardering on the line between 55 and 57 and that's exactly what I discovered when I was ordering mine. So I called Finlay and asked him (UK Rep), he stated "57". Finlay also explained that due to the way Rose measure their frames 57 is the same as many other manufacturers 56. However I still wasn't sure.
So I got the frame measurements for the my 56 Specialized (which was perfect for me) and compared to the Rose 3100 in 57. They were almost identical and that made the decision for me. Ordered 57 and it was perfect.
Having said this many top-tier riders prefer a small frame. Eg Cav. So you can definitely ride smaller frames without issue if that's the way you prefer. It's less comfortable, but looks "pro" with bars low and seat high. Probably easier to get a pro aero position with a smaller frame size, but my 3100 is more of an endurance bike than a pro racer, so the slightly more relaxed position suits well.