you're fine to primary ferment in larger bucket. I've been doing 3 gallon batches in my 5 gallon fermenters recently. Size does make more difference if you choose to do a secondary fermentation (where oxidation of your fermented beer can come into play). You'd be fine with the bucket marked to 5 gallons, but if you find one marked to 6 or 6.5 that's better. Remember when you ferment, you'll lose some volume to trub loss, so to end with 5 gallons of beer, you want your volume at the end of your boil (and into your fermenter) to be slightly higher, around 5.25gallons or so, which may make the "5gallon marked bucket" kinda tight.