First, don't use shoe size to determine skate size. Use a brannock made specifically for sizing skates. Your local hockey shop should have those. People wear different sized shoes for different reasons. For walk around shoes, I wear a half size or so up, because they're generally more comfortable. If I'm looking for shoes for sports, I want more form fitting. My shoe size is 2.5 to 3 sizes larger than my skate size.
Second, once you have your foot measured out by a brannock made for skates, you have a start point. After that, it's about trying on skate after skate, comparing, contrasting, until you find one that's comfortable and formfitting. This is where you spend most of your time. Don't just put on the skate for a few seconds, you need to leave them on for a bit and get a feel for what the skate feels like after the liner has warmed up. Generally, you want to err on the side of a bit smaller vs a bit larger. You can always punch and stretch skates to finetune fit. You can't do the reverse and shrink skates. For big skates, things will only get worse as the liner wears and compresses, making the skates even more roomie