size of heat lamp

I had a ZooMed 10'' clamp lamp with a 100w ceramic heat emitter, that kept my 2x3 c&c cage cooler than I'd like but still within a safe range, but it stopped working, I returned it and prepared to order a new one. In the meantime I purchased a 5.5'' ceramic lamp (also ZooMed, rated for up to...

So I found a heat lamp (just the dome) up in the office and I am pretty sure it is a 5.5... Is that O.K to use or an 8.5? I guess some people say that the bigger the better, but why? What makes a difference if I use a smaller one over a bigger one, would that be OK? If I need to get a bigger one...