Presumably, they are at the size they were downloaded from the camera. Personally, I would keep them at that size for best quality - specially if you ever want to print from them. A number of email programs will re-size attachment photos or get a simple picture resizing program for email purposes, but keep the originals (so save the re-sized version using a different name).
Simple progs like irfanview, in the ultimate list sticky, will let you resize. You can also increase the jpeg compression to achieve smaller files but you will also lose quality.