Does Fermentor Size Matter?

I am making a 1 gallon kit and using a 2 and a half gallon bucket as a fermenter. Does it make any difference in fermentation or should I use a smaller fermentor?
I brewed yesterday, airlock in place, but haven’t seen any activity so far.

That will be fine.

Fermentor size and geometry makes a big difference - if you are a commercial brewer making many barrels of beer per batch.

For a home brewer, the key is just to make sure your fermentor has enough extra volume to handle the foam produced by the fermentation. Otherwise you’ll end up with a mess. So 2.5 gallons for a 1 gallon batch is fine; in fact I wouldn’t recommend anything less than 1.5 gallons minimum. And a 5 gallon batch works best in a 7-8 gallon bucket.