Handy little image viewer and manipulator. To do what you want:
You can then chose the size by pixels or by percentage of the original, it also allows you to preserve the aspect ratio (W x H so it is not skewed).
Handy little tool
VuePrint is a small file viewer/manipulator that has the tightest(meaning highest quality in smallest space) jpg compression that I've come across. www.hamrick.com. It's freeware, I think. Or maybe it is for me, since I bought a license long ago.