The Genesis website lists the sizes in inches, but some other sites have them as S/M/L – which is which please?

Edit: just to clarify, I mean does a medium equate to a 17.5" or a 19" (and so on)?

Think sizes have changed a bit-I've 2 older frames O6? and 08 + both def labelled 19" Med + fit me v well at 5' 10"
If you compare top tube length though these seem to equate to a 17.5" in current (09) range ie think all around 585mm.

My 17.5" actually says "small" on it, but it's an 08 frame. For 09 they realigned their sizing with other companies, and the 17.5" became labeled as a medium, and the 19" a large.

FWIW, dunno how tall you are but I'm 5ft 11, slightly long in the leg and short in the body for my height, so went for the 17.5" version. I could also ride the 19" version, but the 17.5" is a bit more chuckable, and with a 410mm Thomson post I've got plenty of seatpost still in the frame. There's only 15mm in the top tube lengths though, 585mm for the 17.5" compared to 600mm on the 19" so not much in it.

5 10 on a good day and the 17.5 is spot on for me