For wine kits, a 6.5 gallon carboy for primary should work fine. I do all of my primaries in buckets, though. Sometimes those kits have packs of grape skins, or oak to add, in primary. Getting it in there would be hard, and getting it out would be even tougher. Cleaning it would probably be a pain, too.
I use a 7.9 gallon "ale pail" for both wine and beer. It works great for a wine primary!
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