In reply to joepye:
One thing to bear in mind is that different shoe brands use different sizing systems, usually either US men's, US women's, UK or European. They often display several numbers giving the size in all systems (eg Euro 39.5, UK 6, USM 7, USW 8) but usually only one figure is the actual size (often shown in larger print); the others are nearest approximations. To avoid the possibility of completely missing a size in the translation process it's a good idea to get clued up on which brands use which systems (and whether they include half-sizes - most do), and then stick to the relevant system when asking for sizes to try on. For example, don't ask to try on a Sportiva Katana (clue - they're Italian!) in a 6, and then a 6.5 if that's too tight; ask for a 39 (labelled UK equivalent 6), then a 39.5 (labelled UK equivalent still 6, although obviously bigger), then a 40 (UK 6.5). In this particular scenario it would quite possible if only conversing in UK sizes to completely overlook the existence of the 39.5, which might just be the ideal size. This duplication is due to the fact that the different systems use slightly different size intervals, so don't exactly correlate; when converted into the nearest equivalent sizes (or half-sizes) there's inevitably a periodic "bunching" effect.