If you are using Windows. Paint should be able to change the size (with the image open, press ctrl-a, then right click and select resize). It is under the Start menu, Accessories (though should be able to right click the image file and select Open With). Otherwise, GIMP is a free image editor.
http://www.gimp.org/Finally, I think you can do something like
[img width=200][/img]
or similar, but that isn't ideal.