1 gallon, so that you change it everyday. It will grow slime inside if left too long, like if you get one too big. Save the 3 or 5 gallon when you take a short 2-3 day trip. Be sure to hang it or set it on a platform, to keep them from kicking stuff into it.
With feed, you can look into wooden auto feeder building plans, that will hold a whole bag. I have a garbage can by the coop I store my feed in (holds 2 bags) and 2 dog bowls for feeding, one inside, one outside. A scoop into each bowl, once a day, for 6 chickens. I don't like having large amounts of feed accessible overnight to the sneaky rodents.
If you ever add ducks, you'll need the 5 gallon and then some. LOL
The ducks I have, get a 3 gallon for 4 ducks, and that actually NEEDS changed everyday.