2.5/3 gallon batch in a 5 gallon keg

From an oxidation standpoint, as long as there's no O2 in the headspace, it won't matter how big it is.

 

Other than some extra gas lost during "burbing" it doesn't take any more CO2 than a full keg. Filling the large headspace of a partial keg is actually where you end up if you had started with a full keg. Carb time is less than half.

 

Probably half of my brews are 2-3 gallons. I do use small kegs for these batches. I got the small kegs out of concern over space, not oxygen. I have four taps but can only fit three 5-gallon kegs on the floor of the keezer. I can fit one smaller keg on the hump where the compressor is, allowing me to utilize the 4th tap. Prior to getting smaller kegs, I used my 5 gallon kegs to hold the smaller batches, and detected no problems with oxygen. Prior to kegging I fill kegs with sanitizer and push it out with CO2, which I believe goes a good ways towards preserving the beer; if you do this, I think you'll be fine. The reason I decided to get more small kegs was to save on CO2 and sanitizer when I do these purges for small batches. I now realize that I could add more taps to my keezer that could be utilized if I stack small kegs. However, I resisted the urge.