I have a 6.5 for primary, though we just put our second batch into a plastic primary because we still had another batch in the 6.5.
That said we just got six 5 gal carboys for $15 ea off craigslist! They're all a bit different though, as they're old water bottles. The necks are a bit narrower than the 5 gal I already had that I'd purchased at my old LHBS when I lived in Albuquerque. Probably about four of them will be used as secondaries for ale, but as I understand it these will make good primaries for mead and cider, and we're looking forward to doing some of those too.
We eventually want another 6.5 so we can do two batches at a time in primary.
Don't forget a blowoff hose for your new carboy, and enjoy!