Primary for 5.5-6 gal batches should be in a 6.5 carboy (extra wort volume for sediment loss). Secondary and aging in the 5 if necessary. I have used a 6 gal for primary, and needed a blow off tube to contain the reaction. I like the 6.5 gal best because they are larger diameter cylinder, you will read that commercially, short wide fermenters are preferred to tall skinny ones.