Fermenter size vs batch size

Will 2.5 gallons produce enough CO2 for me to safely ferment in a 6 gallon carboy without oxidation effects?

I do 2-3 gallon batches once in awhile in a 6.5 gallon bucket and have never had any problems. You’ll be fine.

Yep, shouldn’t be a problem at all. Brew away.

I’ve done it this way for a long time. The one thing I’m starting to wonder about though, is whether I’m getting a little oxidation from it that is costing me some malty flavor. Im thinking of racking to a 3gal secondary before the beer is done fermenting but after the peak, and making sure I get plenty of yeast racked over so it doesn’t stop fermenting. This way I minimize the head space without having to use a blowoff tube and all the mess that goes with it.

Yup. An average-gravity beer will produce about 20 volumes of CO2 per volume of beer.