Frame Size

[jpringle819]

I am looking at getting the wife a bike and Halfords have a really good deal on a carrera vengeance. The guy in Halfords is suggesting that the 20" frame is the one to go for but I think it may be too big. I am 6'2" and have ridden an 18" Kona for a few years but now have an XL Cannondale. My wife is 5'10" but has a longer inside leg than me, when sitting on the 20" it looks fine and the bike does have a sloping top tube so doesn't seem as big as some other bikes.

Should I trust the guy in Halfords? Does anyone have a good way to work out the correct size?