I use a 5gal carboy for some beers that I might have a reason for, but generally 6.5 is best.
My most recent beer was a lambic... I even exceeded 5gal too, filling it up maybe an inch from the neck, but of course, I used a big 1-inch blow-off tube stuffed snugly into the opening.
Because the lambic blends already have a lower yeast cell count than typical vials/smackpacks in order not to outcompete all the bugs, and also because in order to maintain that balance, starters should not be used, I already knew the fermentation was going to be a bit underwhelming. I had a bit of blow-off, but no noticeable loss of volume. Plus, lambic is supposed to remain in the primary the entire time, which is a long time (this one won't be ready until 2014!), and unlike Flanders Ales, oxygen should be kept to a minimum, which is why I chose to use a glass carboy (oxygen-impermeable) with a volume of 5 gallons (minimal headspace and exposed surface area, since it's near the neck).
So because I wanted minimal headspace, for the reasons discussed above, I was set on using a 5gal carboy, ready to accept any loss of volume to blow-off. It certainly helped that I knew the fermentation wasn't exactly going to be vigorous, but as long as you use a length of thick, 1-inch diameter tubing stuck firmly into the neck of the carboy, there's really no risk of clogs (and subsequent explosions), so the only real concern is how much beer is going to be lost to blow-off.
If you consider that lost volume acceptable, then there are definitely situations where a 5gal carboy is preferable. But the people here are pretty much right, anyways. 6.5gal is by far the best choice for the vast majority of brews, ESPECIALLY for a new brewer.
