Is my carboy big enough?

First batch of beer has come and gone. It was delicious and I was impressed by how nice it turned out. Lessons were learned and I'm eager for more.

I was able to get a hold of an old glass water cooler bottle and my intent was to use it for my secondary fermentation (I've already got a glass 6.5 gallon for primary). Well last night I was calibrating it with volume markers and at 5 gallons, the level was right into the neck. If I still have 5 gallons to transfer to the secondary, is this enough room at the top or am I going to have problems?

If it matters, I'm brewing the Brewers Best Holiday Ale with the Wyeast 1728 Scottish Ale.

I'd say if it's finished fermenting you could put it in there, Maybe use a Blowoff just in case?

If you let primary run totally to completion, there's rarely anything "interesting" going on during secondary - unless you make some kind of addition that provides fermentables (eg: fruit)...

Cheers!

As long as you take consistent hydrometer readings and conclude that fermentation is over, then the less headspace the better. Too much space can cause oxidation.

Nothing happens in secondary. Once your krausen falls, you can transfer to this sucker no problem.

With a blowoff and fermCap-s, you can primary in it too. (Hey, if it works with cornies....)

I appreciate all of the advice, so thanks. One thing is that in the brewing instructions, they recommend transferring to the secondary before the primary fermentation is complete. They say "when the fermentation slows (5-7 days), but before it completes". As a new brewer, I don't necessarily understand why this would matter if you waited for completion. I do realize not much is going on those last few days anyway.
I only used standard airlocks with my first brew, but I've got extra stoppers and tubing to make a blowoff, so I may try that to reduce the possibility of clogging.

Through all of the reading on HBT and other forums I have come to the conclusion that "directions" given with kits are a basic guideline for faster beer instead of better beer, some people don't use Secondaries at all.

There's really not much space for additions, which is what a secondary is used for. I'm thinking along the lines of dry hopping. I guess spices, etc. would be fine.

Regarding the directions provided, I would just leave the beer in the primary for the extra amount of time. There are MANY posts that provide reasons why, but one main point is that it gives the yeast time to clean up the beer. I will only transfer to a secondary if I want to add stuff, but this is after my normal month on a primary.

just as an update. I did use this carboy for a secondary on an extract Bock recipe and it was completely fine. I just used a standard double bubble airlock.