Originally Posted by archiefl98
I've had blowoff even in a 6.5 gallon carboy from a 5 gallon batch before. 7.9 gallon buckets work pretty well without blowoff tubes. (I know that's not what you asked)
I'd go 6.5. That extra half gallon of space makes a difference.
+1 for archie above.
If you do not have a larger 6.5+ carboy right now I would not rush out to buy one. Worst case scenario is you need to use a blowoff tube, however, if you find a recipe for more than 5 gallons you could always just scale it down.
Brewing software and internet sites abound with help in formulating recipes. A site I like to use is brewersfriend but there are lots more! Or of course you could ask some of us here on HBT! Just my humble opinion!