Best size fermenter for small batch

So, I'm gonna brew a 2.5 gallon batch of braggot, since I've never done that style of mead before.
As I understand it, a braggot will have krausen unlike other varieties of mead.

I have a standard 2.5 gallon carboy, but I'm fearing this will blow off too much. My 3 gal carboy is currently lagering for several more weeks. And I have an open 5 gal carboy.

Should I risk the 2.5gal? (I have no blowoff hose, usually primary in a bucket), or do I use the 5 gal for primary and then the 2.5gal for secondary?

I would just go with the 5 gal. unless you want to buy another 3 gal. If you have a kegging setup just purge the carboy before filling and it should be good.