Ordered a pair of Fizik R4s a size too big. Will it make a difference? About a month ago I ordered a pair of Fizik R4 shoes, they recently came in and I was ecstatic that that looked so good IRL. So ecstatic that when I saw the 12 on the side of the box I didn't realize it was for the UK. I normally wear size 12 or 11 1/2 depending on the brand. Will wearing a complete size larger make a difference in terms of how many watts I can put out or put me at a higher chance of injury?
Everyone will agree that it is best to have shoes that fit properly.

If the shoes don't fit perfectly, it's better to have shoes that are a bit too big, rather than shoes that are a bit too small. As we warm up our feet swell slightly, and if the shoes are too small then the blood circulation can be restricted. That can lead to cramps or numbness. 

The issue with shoes that are too large is that the foot could move inside the shoe, causing blisters or other injuries, and reducing power transfer. 

If wearing an extra pair of sox and cinching the shoes tightly stops any movement, then you'll be fine.

One concern you may have is cleat position. With shoes that are only one size different, I don't think that will be an issue; you should be able to position the cleats as usual. And research has shown that our preferred cleat position is largely a matter of tradition, and make extremely little difference for power transfer.