Hi, i m thinking of building a KM frame, but i dont know what size, i m 5'9 ,32 inside leg, i was thinking maybe the 18" but i don t want a short top tube, i d like to be a bit stretched.

Thanks

If you are worried about TT length then take a look at some of the AM hardtail frames with a lower, but longer TT (On-One 456, Ragley Blue Pig, Evil Sovereign, Cotic BFe, etc..).
Maybe the KM actually has a longish TT for height, I didn't actually compare geos. I just wanted to point out there are frames out there meant to have a lowish TT height (ST length) while maintaining a comfortably long TT length.
For rougher riding an 18" frame may have an uncomfortably high TT for you at 5'9. I'm 6'1 (never quite sure about my true inseam) and ride an 18' Evil Sovereign, which fits me almost perfectly, but then again fit is very subjective and depends on what type of riding one does.