1) In reality a fermenter is a fermenter is a fermenter.....glass, plastic, carboy, bucket, jerry can, keg, milk jug, ceramic crock, glass hurricane jar, stainless steel or plastic conicals, pet food storage vessels (vittle vaults), HD or Lowe's buckets, frosting buckets, water jugs, the old Mr Beer jug...All of those and anything you can think of, all work perfectly fine, and have been used by hundreds if not thousands of brewers...
No one type is better or worse than any other...good beer or crappy beer can be made in all of them, dependant onthe brewer, NOT what it's fermented in...
It's really just a matter of preference, nothing more....There's tons of arguments about glass vs plastic and whatever, but they're nothing more than internet masturbation....the yeast don't give a crap what kind of container the wort it is swimming in.
It really isn't rocket science, it's really about using what works for you.
It won't have an "impact" on the beer one bit.
2) I like a minimum of 1 gallon headspace in my primary fermentors. For a 5 gallon batch I like a minimum 6 gallon vessel or larger. In terms of carboys, plastic better bottles come in 6 gallons and iirc glass carboys are 6.5 gallon.
Personally if I were doing a 5.5 gallon I would go with a 6.5 gallon. But I do 5 gallon batches. So a six is fine.
If you're in doubt, use a blowoff tube instead of an airlock.
