Are my cycling shoes too big? With 3/4 inch of room My shoes came in the mail and I have about 3/4 inch of room to the end. On a short ride I didn’t notice any slipping when I had the boa tightened all the way up. I also don’t have any hotspots or blisters. If my shoes are too big will they affect my efficiency or are they fine?
1.9cm (3/4 inch) is a lot of room. I assume you looked at the imprint of your feet on the insole.
I have about 1cm room in my Specialized Expert shoes and I regret not getting them at least half a size smaller.
Things which annoy me about my too-large shoes:

Walking can be awkward because my feet float a bit and especially ankle stability is just not there because the foot can twist inside the shoe. I’m also more likely to trip over the tip of the shoes.
The shoe’s tongue goes too far up and starts biting into my feet after some time. Fortunately it’s possible to push it further down.
It’s easier to hit the front wheel in sharp turns.

Riding itself is fine since they more or less just have to provide a secure platform for your feet. I can also move the cleat far enough backwards.
There is a slight advantage in cold weather because you can easily wear two pairs of socks or waterproof socks.