I'd concur with that. I'm 5'10.5" (33" inseam) on a good day and ride a large Rail with very little seat post extension. The large has a 170mm dropper whereas the M has a 150mm but can take a 180mm One Up dropper.How tall are you?
I'm 5'10" and the L Rail felt perfect when I put some test miles on one. I'm usually in between a M and L.
5'9" or taller I'd go with a L
5'8" or shorter I'd go with a M
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