If you decide to secondary (and that’s a whole nuther discussion), Always opt for whatever will give you the least amount of headspace if you have a choice. So if you’re doing 5 gallon batches(for example), a 5.5 gallon carboy is a good size.I only do secondary when I’m aging a lambic, and I use glass because that’s what I have and because it’s a sour and will clean well. For standard beers, plastic is fine for secondary unless you’re maybe aging something like a barleywine for longer than a year.
Cheers,
Ron