20 foot boat, perfect, we use a 30 or 35 gallon trash can, get the heavy duty stuff, not the real cheap ones, they will loose their shape, tear, etc. If you have the room a 55 gal barrel is great. Ours sits in the corner between the transom and the port side, once there is some water in it it doesn't budge. We plumbed in a quick connect on the side down at the bottom for water intake from the salt water washdown. We added a piece of flex pipe a couple of inches down from the top for the overflow. This flex pipe hangs overboard so that the overflow water does not enter the bilge (bad mistake if your bilge pump develops an issue
). You can go with a heavier pump, but we find the washdown works fine. You have to keep the cover on or the bunker will come rocketing out of the can. You can cut a hole in the center of the cover and make a flap for easy access. This set up will hold 12- 15 large bunker alive all day, a 20 gallon tank will not keep as many bunker alive for any period of time.
