Is ti OK to use a 6.5 gal glass carboy for a secondary or would it be best to use a 5 gal for a 5 gal batch? Would the increased head space in the 6.5 be a risk for oxidation, or will the CO2 in the beer push the air out?
1. its not secondary 'fermenter'. no fermentation should be taking place. its a secondary 'vessel' or 'clearing tank'.
2. I would not be extremely worried about using a 6.5gal carboy for secondary as long as I treat that carboy like a newborn baby....really gently.
surface area exposure to air isn't a guarantee for oxidation...but agitation is what creates gas exchange in a liquid.
Ideally, invest in a 5 gallon carboy at some point, and try to limit the 6.5gal to primary.