Too small will prove frustrating to use, too large will get tiring to use after a short while. You may be using it a lot, make sure it is comfortable to work with and is equipped with a handle. Few bailers supplied with rafts are any good in my experience. Tape Industrial or military grade duct tape is best. Make sure it sticks well when wet.
Some brands do better than others.
Effective for small leaks above the water line and just about the most useful material known to man when improvising or making repairs. How many depends on how many valves are in the raft, one for each. Many topping valves have integrated plugs attached, but carry a couple spares, they tend to be torn off and lost. Most also don t float. Make sure you add lanyards.