Hello, I need help on toolbar buttons size.

I'am actually working on a 24" DELL Monitor set at maximum resolution that is 1920x1080 and the picture below (hope it works) shows how button icons look like: as you can see, they are quite (too) big and I'd like to know how to reduce their size. I've tried to modify them through Icon Explorer, but I did not manage to do anything.

Can someone help me? Thank you