Are you sending them from iPhoto? If so, choose Share, then Email. The pop-up window has a drop down menu to allow you to change the size.
Choice of Small, Medium, Large or Actual size.
I've just had a look and a 2.1Mb actual size is reduced to 424Kb for large, 125Kb for medium and 65Kb for small.
If you are using another photo processing application, they will almost all do the same thing, somehow.