5.11-6ft here and ride mediums, I’m even more on the divide between frame sizes though.
My taste seems to go in cycles though. I used to ride medium XC frames (~590mm effective top tubes) with long (110mm) stems, then went upto larger frames (~610mm) and 90mm stems, then went back to 110mm stems. Which is definately a large from most manufacturers.
But conversely I’ve just bought a medium Pitch, which while short, is nicely ballanced which more than makes up for the more upright position when sat down.
At 5.10 I’d say you’re definately medium, get some demo rides to confirm if you’re not sure, most brands have demo days through the year where you can try as many bikes as practicaly possible for free. I found that I got allong well with a medium Pitch, but needed a large reign to feel comfortable, so it’s not even limited to the same size as the mesium reign and pitch measured up almost identicaly.