These are 5 gallons or 5.25 gallons depending on the maker of the bucket. I use these and the snap on lids with no problem at all. I ferment 3-4 gallons of wort in them with no trouble. I have about 10 of these because the biggest size I brew is in the 8-12 gallon range and then I split these into 3 smaller batches to try out different yeast strains, dry hops etc..
I know one poster in this thread mentioned being concerned about the snap on lids but I have never had any trouble with them and have used each bucket multiple times. Another option would be to buy one of the Walmart buckets and then get a lid from Lowes with a snap ring gasket seal to use. These lids are $2 at Lowes. I figured that part out later so i will move to these at some point in the future but don't need to yet.