After installing XNView:
1 ) navigate to the directory where your image is located
2 ) open the image
3 ) select "image" "resize"
4 ) make sure the "keep ratio" box is checked, I suggest Lanczos resampling be selected in the lower pull-down box. You can either use a pre-set size from the top pull-down, or choose "custom" and type your own dimensions.
5 ) choose the size you want it to be
6 ) click on "file" then "save as" and select a NEW filename or it will overwrite your original.
XNView also happens to be my favorite image viewer. Did I mention it's also free?
It's also multi-platform.
You'll find most of the software I recommend falls into these categories.
I'll track down XNVIEW and get it loaded.