I think this is a really important point. I'm usually happy in my Shimano Tri shoes sized 44 - UNTIL mile 38 in 85+ degF., and the toe pain comes in bigtime. Now problem solved in Giro 46.5's for 3 hours plus, no problem. Bonus to have Giro's 3 sizes of arch support.

As much as I hate paying retail, mail order shoes are a risky proposition. Not only is there no standardization among mfr.s, but within the same company for a given size. Not sure about cycling, but running shoe fit can vary widely depending on the last. I've bought 2 pr.s of la sportivas, both 10.5Ds. 1 felt like foot binding and my feet were swimming in the others.
As said, feet swell as the day progress, especially if bearing weight. Always recommended to try on new ones late in the day.

Got my first pair. Fizik in 43, and fit great. high arch though. It was painful beginning of my first ride, but much better by end of it. Hope it will be better after two to three rides. Getting another Fizik in same size, and a Giro in 43.5. Keeping fingers crossed.