Effects of jockey wheel sizes & types
Main gain is less friction when using ceramic ones and less friction on the transmission when using up to 15T pullies. This translates into couple of watts after a big number of km's.

If you are into pro racing or you are a really weight weenie or you just have money to burn, then you can go and change both pullies if you want to. From several studies, at very high performance you inded gain some power from using larger pullies, but that is really almost irrisory and only really valuable at really high performance levels, say pro riders at Tour de France, Vuelta, Giro, etc. Other than that, ceramic ones have less friction so is supposed to save you some watts on long rides, which translates on some miliseconds on the clock after couple hundred km's.

If you check prices, good ones are up to 400 usd...so, what you will do, change pullies or fully upgrade your transmission? I'll go for the 2nd option ;)