I personally use a 5 gallon glass carboy for secondary. Using a carboy that is as close as possible to your batch size in secondary minimizes surface area exposed to possible oxidation.
Ideally, you shouldn't have any signs of vigorous fermentation in secondary. The bulk of that should have been accomplished in primary fermentation. You shouldn't have to have room for extreme krausen.
A 6.5 gallon is excellent as a primary fermentor, specifically because it does allow for massive krausen during high SG fermentation. You could get away with a 6 as a primary in some cases, but there's no way I'd use it for secondary. There's just to much headspace unless you do a CO2 purge.