5'10" = at least a M/L for me...
Interestingly, Litespeed does not have a fit chart (not that I could find anyway) of general guidelines for height/inseem matches to frame sizes. I relied on the bike shop to fit me, but I think they may have been biased by the sizes of their particular stock at the time I came in.
I'm just over 5'10, 33 inseem, and I just bought a M/L Teramo (nearly exact same specs as the Siena (except I got more g's/$ !). After a few days of short rides I am wondering if I should have gone with the L, primarily to get more headtube length (+2cm). With the M/L (56cm TT), I have 35mm of spacers (nearly the limit for a CF steertube) under a 100mm +10 degree X2 stem and my bar to seat drop is still 2 inches - which is a bit much for me due to a failing C5-6 disc. It seems to me like I am making up for a frame that's too small by adding so many spacers and such an aggressive stem rise.
I could stand to run a bit longer stem, but was thinking it would be better to stick with the 100mm and get another 10mm of stretch by going to the L (57cm TT), where I could get the bars up just a bit more on the longer headtube with less spacers than I am running now.