I wear a 37.5 Solution and a 6.5 mens Project. For reference, I wear a size 9 street shoe.

I have tried on the solutions and wear about a 37 or 37.5 (I can't recall exactly). In the 5.10 shoes I do have, I wear 39.5 (39 if i really want to cram my toes, so usually 39.5). La Sportiva's sizes run smaller than 5.10 (and Madrock, where I'm also a 39.5), in my experience. I should mention that I'm a 38 in street shoes, so I size down for La Sportiva and up for ... everyone else I've tried. I should further mention that I've never tried the Project (though I'd like to), and it may well fit very differently from the other 5.10 shoes I have.

Five Ten uses men's US sizing. Usually I get my shoes one half size to one full size smaller than my men's street shoe size. If you have extremely wide feet, you might want to get the same size as your normal hiking shoe size. These are really soft, so should be pretty comfortable right away and might feel a little bigger than a stiff shoe.