I’m 5’10 and always ride a Large from Turner – have had six 5 Spots and one Flux now.
Have ridden several Mediums and for my size they just feel too small.
Over the years as geometry has changed my stems have reduced from 90mm -> 70mm -> 50mm, but bars have got wider, forks taller and steering slacker, so it’s not exactly an apples-to-apples comparison.. more an observation/commentary on how geometry has changed between 2005 and 2011, not just on Turner but all bikes really.
My Flux was a Large, 2009 DW model and I ran a 70mm stem.
At 5’10 we are what Dave Turner calls “Tweeners” – right in between a Medium and Large. Best to bag a test ride before choosing, as so much depends on what you’re coming from before.