headspace in secondary

Is the headspace in the secondary important ?

I mean, is it ok to use a 6 gal carboy for secondary for a 5 gal batch ?

you can because some off-gassing occurs when you rack it, leaving what should be enough of a CO2 layer on top of the secondary.

Why do a secondary at all? Are you adding more yeast? or are you using this as a clearing/bright tank?

Most of the time, I stay in primary for 3-4 weeks, then straight to keg. Kegging has made me make and consume more beer ! No more "Hmm- 9 bottles left, time to make more" scenerios...

Last week, getting ready to keg and the brown ale was looking cloudy...I had already kegged 1 batch that morning, so I had all the equi[pment out..figured I would clear in secondary...was there for 1 week.

I kegged that batch today, and came out very clear...quite a bit of yeast at bottom..

I wouldn't. While Android is right - there is some off-gassing during the racking process - there is always the risk of O2 introduction. If you have an empty 5-gallon carboy, why risk it? If you don't, it's a good idea to purge the 6-gallon carboy with CO2 if you've got a tapping system (in fact, it never hurts to purge carboys of any size).

Cheers!

Bob