Head space in fermenter

I’ve been using 3 gal carboys for 2.5 gal batches (NB 5 gal recipe kits that I split).

I want to try a BIAB 3 gallon recipe. What is the best size primary fermenter for a 3 gallon batch?

The krausen in my 2.5 gal batches pushes the limits of the 3 gal carboy so I’m sure I need larger but is a 5 gallon carboy “too big” and results in too much head space?

Thank you

5 gallon carboy should be fine.

once the beer starts fermenting it will create a layer of co2 that will protect it from oxygen.

[quote=“S.Scoggin”]5 gallon carboy should be fine.

once the beer starts fermenting it will create a layer of co2 that will protect it from oxygen.[/quote]
+1
I’m currently brewing a 3 gal. batch that will go into a 5 gal. carboy. I’ve done this several times.

head space is more of a concern in the secondary, where you fill an environment that’s full of oxygen. You can lower the risk of oxygenation in the secondary by first purging the carboy with co2

I was debating doing this at one point. How much of a pain is it dividing ingredients in half (LME)? Do you repack the dry ingredients in Ziploc bags?

Really no problem dividing at all if you have a kitchen scale.
I weigh and divide the DME into two small tupperware tubs.

If the LME comes in a single bottle I just guess and put a sharpie mark on the bottle. You can weigh the bottle but my kitchen scale only goes to 5lbs.

Hops are easily weighed and divided into cups for immediate use or ziplocks for later.