Use the biggest bucket you can handle. This can save you a lot of trips to and from the sink. Use this bucket ONLY for your aquarium and don't ever put any soap or other type of cleaning solution in it. Soap residue will KILL your fish quicker than anything. No matter how hard or how long you rinse your bucket, there will always be traces left in there.
Plastic gets little nicks and scratches in it the the soap can stay in.
So buy a new bucket from the store and never use an old one that you perhaps used to wash your car with. You should be OK if you get one from a restaurant that had pickles or something like that in it. You wouldn't have to worry about soap in there. Once again I stress not to use any types of household cleaners in your tank. One of the best cleaners you can use is salt.