If you crop an image, you are just removing some of the picture around one or more of the edges. It will be a fraction smaller merely because there are less pixels as you cropped some off but not much different Mb or kb size to the original. Cropping does not reduce the quality.
You can then resize the picture to 640 x 480 pixels (max). This will reduce the kb size.
You can then reduce the quality of the image which will drastically reduce the kb size, depending on how much you reduce it.
eg. My original photo would be approx 3.5Mb.
If I just resized it to 640 x 480 then saved it as full quality it would be about 250 - 300kb
If I then reduced the quality to medium (or 5 on my scale) for emailing, it would be 80 - 100kb.
For those who get the AW, the cover photos are full quality but resized to 640 x 480 so are approx 250kb. The pics on the inside, being so small, are done at medium quality so are about 100kb each. Makes the file size of the document smaller and prints faster.
If a pic is just for online then 100kb is fine. If you want to print it out the same size as a normal photo then 250+ is enough. If you want to print it out A5 or larger then keep it full size.