Yeah. You could use a bigger one in a tank that long but the 24'' will be good enough. The goal is to provide a beam that is at least as wide as the length of your dragon so they can bask evenly. If you have that covered then it's enough. It's good to have an area of intense UVB while providing some other areas with less or no UVB so they have choices available.
Thank you Cooperdragon. I thought it was enough. Another forum was saying it was not enough. Still trying to figure out what is wrong with Mordecai. Vet is stumped at this time. Might get and atadenvirous test done just to be safe.