In reply to Wyre Forest Illuminati:
Years ago we stocked about 30 models of rockshoe and I fitted from a UK4 to 7.5, depending on mode on my side 7.5 foot. LaSportiva pretty much always needed going down at least two full sizes from your street shoes, most other models about half or one size.
A knowledgeable shop should be able to make a good guess of what size X offers a comparable fit to Y, but often you see people going bigger to try and make a shoe that is simply the wrong shape fit their feet. The shop also should have a good idea of which models will have the most stretch (generally unlined leather) and be able to advise, IME though all sorts stretch to some degree and the tighter you fit them the more they stretch.