On that bike it depends on what seat angle you intend to run--I am your size but have long arms, I ride a 58.5 top tube on the road and 56.4 on my specialized transition. I'm looking at a 58 P2SL, with the seatpost forward I'd have about a 56 top tube, which would just clear my elbows, but I'd have a steeper seat angle than I have now and more bar drop for lower shoulders.

I know a rider of my exact same size who is a very good time trial rider who is running the size 56 P3, but has the post in the flipped back position because of the UCI regulations.

I'm going in to get measured up this week, I threw a tape measure on a 58 last week in the shop with just bars on it and it looks to be in the ballpark, I think a 56 would be too small unless I wanted to ride a very slack seat angle. With a small frame you will have toe overlap, but who cares.

It comes down to seat angle for you.