If you're pouring from the bag into the trash can I think you could get away from a 10 gallon for each type of feed.
I store mine (still in the bag) in 20 gallon trash cans in my garage. With just one bag it's roomy, short enough that it's easy to reach the bottom, etc. At one point I had one bag of hay pellets and one smaller 25 lb bag of flax seed both in one can. If I were to dump the contents of the bag into the can I'm pretty sure I could fit at least 2 bags (100 lbs) of feed.
ETA- even better, I remembered that Triple Crown (the feed manufacturer I use) publishes the weight per volume of their products. Their 30% Supplement is 1.25 lbs per quart, which means 50 lbs is exactly 10 gallons. So, you might want to have a little bit larger can just so you have some wiggle room.
Last edited by verona1016; 12-09-2013 at 08:04 PM.