I was using 6.5 gallon glass carboy but recently upgraded to 7 gallon Fermonsters (currently have 6 clean and 2 sour) and am loving them. I normally have ~6 gallons going into fermenter and have yet to have the need for a blow off. With a 6.5 gallon carboy I almost always needed one.
I know it is too late since you posted you already pitched the yeast but with it being a mead yeast there is a good chance it will not be able to consume the longer chain sugars that are in beer. So there is a good chance that the beer may not ferment completely. So you may need to pitch a sac...