blow off loss and secondary fermentation

I started a imperial stout on Saturday and within 8 hours it was going nuts. Yeast starter ftw. I'm doing primary in a 6 gallon better bottle and from what I can tell, I've probably lost .5-.75 gallons from blow off.

I'm planning on transfering to a 5 gallon secondary but a little concerned that since I now only have 4.5 gallons, I'll have too much head space and the beer will oxidize. Should I be concerned about this (I plan to age in secondary for at least a month)? If I should be concerned is there any way to bring the volume up to lose headspace, like adding sanitized marbles or something?

I have an IPA that due to hop absorbtion and blowoff loss only made about 4.25 into the secondary. It's been in there 3 weeks and it's fine. I figure I'll only get about 4 gals into the keg. You'll be fine with 4.5 in a 5 gal carboy.

Sounds like you should be okay.