I think most digital cameras have an option to change the size. But I wouldn't change it. You're going to want all those pixels if you decide to make prints from the digital pictures. Also sometimes you want to crop a picture. Having a lot of pixels guarantees that a cropped picture will still be detailed enough to look good - and be good enough to have prints made.
What you might do instead is to use a program to make smaller copies from the originals.
Irfanview is free and well thought of for this sort of thing.