I use 7 gallon fermenters for 5 gallon batches. I just picked up a 14 gallon fermenter for 10 gallon batches.Personally, I probably wouldn't put more than 10-11 gal in a 14gal fermenter.
I brew 5 gal and put in 6.5 (or is it 7?) gal fermenter and have gotten blow off.
See how it goes from there.
There’s probably a little wiggle room, but I have seen somewhere that you can batch about 75% of your fermenter capacity.
Looking forward to hearing how it goes for you; be sure to keep us posted.
Cheers!