That does appear to be a rather small pot for a tomato plant. The container underneath to hold the water will definitely help; they do like lots of water. What variety of tomato is it?
Oh, and your broken branch stands a very good chance of rooting and making a new plant for you. Not sure if the fruits will survive, but you should end up with a new plant. I once broke off a cherry tomato plant that I was trying to transplant, right at ground level. I stuck it into a pot of wet potting soil and it rooted for me with no trouble. But that was early in the season.
