Ah - Yup "Vintner - Know your Yeast"It wouldn't be enough for the five gallon wines I've made. I have both a 6.5 and a 7.8 gallon bucket (and three five gallon buckets for my 3 gallon wines) for primary fermentations. My 6.5 regularly foams up to the top (and more) with a little 5 gallon Fontana kit. The 7.9 was barely deep enough. Some yeasts are lower foaming than others but if you plan to use Red Star Premier Rouge, get ready for some massive foaming action. It makes reds taste great but good grief it foams up a lot.
It might be a good idea to investigate which yeasts tend to foam up and under what conditions.