On my road-bike I ride a 56 cm frame. That gives me a toptube length of 555, and that is quite comfortable. Now I'm looking at a Jamis Nova, and thought I would need a 55 cm frame, since that's what my road-frame is, but in looking at the specs, It appears that the 55 cm frame has a 570 effective top tube length. Does this mean I should drop down to the 53 cm bike (555 eff TT)? I'm 5'11" with a 32" inseam. Would the 53 cramp up my cockpit somehow? I'm pretty confused. The stem for the 55 is 6deg rise 120 mm, and the stem for the 53 is also 6 deg rise but 100 mm. Should I just get the 55 with a shorter stem or go with the 53. Another reason to go with the 55 is the 44 cm handlebars vs 42. I think 55 is the way to go, but I'd like some input from others. I don't have either available for a testride, only a 51 is in the shop. Which bike should I order?

in general, size the bike by the TT. And yes, most people end up going down a size from their road bike. The 53 sounds right for you, it gives you the same TT as your road bike (so with the same stem as your road bike you will not be cramped up). Stems and bars can always be adjusted, frames cannot.

i'd go with the 53. many cross bikes have higher bottom brackets so seattube lengths shrink.
size with the toptube and in this instance go with the same length stem as your road rig.
even if the bottom bracket isn't higher you'll still benefit from more standover on the dirt.
that's it. get the 53.

I sat down with my roadbike and a measuring tape. and the geometry chart for the two bikes. 53 is the way to go, since if I got the 55 I'd need a REALLY short stem to duplicate my riding position. I think that may impact the handling characteristics, so I'll go with a 53 and show a little more seatpost. Thanks for the advice.