Picasa is a great, easy to use photo manipulating software package that is free from Google. It has a number of cool features, but resizing images is a little tougher to figure out.
When you first install Picasa, it will scan your computer for pictures. Select the picture(s) you want to resize and then go to File -> Export Pictures to Folder. Make sure "Resize to" is selected and use the slider or enter the maximum number of pixels that you want in the largest dimension.