For a 5gal brew i used a 5 gal carboy as a secondary .. If your talking primary my understanding is, it's not that important.. it's making so much CO2 in fermentation it'll drive out the O2
I used a 5 gal carboy for my primary. I filled to just where the neck start to work upward. I think next time I am going to add a little more water to the carboy to make up for excess testing of the gravity throughout fermentation.
Using a 5 Gal primary for a 5 gal batch? You must have some huge blowoffs. I have gotten at least one blowoff using the 6.5 gal bucket with a 5-5.5 gal batch. With something that does not krausen like apfelwein I fill to just below the neck. If I were brewing beer in a carboy I would probably fill to the shoulder tops and still use a blowoff tube.
I have a s curve airlock on top. The krausen ring is working its way up the neck of the carboy but this is the third day so I imagine in the next 25 hours its gonna start calming down.