Bushwacked I had the same issue last week. I eneded up asking JonEdwards as well (very nice chap, local to me and can really ride a bike so he knows what he's talknig about).
He has a small BFe but he also has a medium Simple and a Soda! He advised me that if i wanted the bike to be anything other than a play bike, then at my height (5' 11") I should get a medium, whether that be a Soul or a BFe (I was undecided at the time, ended up ordering a BFe)
He said he order his BFe in a small deliberate to compliment his medium other Cotics. Here's what he said:
"I'm 5'10" and usually ride medium Cotics – I have a Soda and a Simple as
well, but I deliberately bought the Bfe as a small, as I wanted a
chuckable bombproof play/dj/dh hardtail. It's actually turned out to be
a chunk more versatile than that – with Pikes (or 2010 Revs) it works
pretty well as a trail bike, so long as the ups aren't too technical
(the bottom gear of 36/32 is the limiting factor, especially on flat
pedals), Lyrics, and it's an absolute hoot DH, especially if it's
techy. "
Hope that helps and Jon, hope you don't mind me quoting you.