when i did my batch, i used the same bucket that i use for my canning pickles. it was a good size, just the standard ball canning set. i would recommend a little bit bigger though because i had to keep an eye on mine to watch carefully for boil over.
on a different note, i did have to add water after my wort was cooked up. this benefited me 2 separate ways. the first was that it didn't need as much room on the stove because the cooked wort was near 3 gallons. also, once i transferred mine into the fermenter, i added about 2 gallons of cold water to make it 5 gallons total. this also helped cool my wort down faster before i added my yeast.
hope this helps, but my experience is from one batch only so far