get some idea of your actually shoe size by getting measured, then be aware that not all shoes of the same nominal size are the same size and what works for one style of cycling might not be best for another. Mainly this is down to the last used. Shimano shoes use a narrower last than, say, Spesh. Having the right width is critical; as Jimbolee says, if the shoe is to narrow you will be in for a world of unexpected pain on a century ride.
For example my shoes; Old, very old, Spesh Rockhopper MTB boots size 45 fine for mtbing all day, agony on the road after two three hours but the pain, outside of right foot, can be switched off by unclipping and wiggling my toes whilst still pedalling. Spesh Tahoes size 46, great for my hourish commute, or arsing about on the MTB, or indeed touring (I stop and walk about a lot "oh look a castle", "oh look a canal", "oh look a bike shop" etc., etc..) but way too flexible for long road rides in the saddle. Same deal as the mtb boots pain wise. Spesh BG Sport size 47; shoe Nirvana, big enough wide enough can ride centuries in them only stopping for red lights. Lake SPD sandals size 44/45, go figure! on paper way too small but being sandals that I wear to go down the shops or out with the slow socials on a club ridse to a pub? Fine.
Shimano shoes in any size... start hurting before I've even done the laces up. That last is too narrow.