There are a jillion ways to resize pictures, but as far as I know there is only one that:
1) Does not require you to open the image file
2) Does not require you to save the modified version of the image.
3) Does not require math.
This application is Posted in #2; and while I use or have used many of the photo editing software listed here, this is what I ALWAYS use to resize.
In other words........."mine is better".