I like irfanview. It's free and has some other toys to play with your photo's as well.
IrfanView - Official Homepage - one of the most popular viewers worldwide
Once you open the file with the program go to Edit then click Resize/Resample and select the size and quality you want. Then save (I recomend saving as a different file so you can keep the original high quality file).
There's other toys in there as well to play with your photos.