This DOS batch script allows you to select a set of images in Windows Explorer and then right-click and choose SendTo > resize_images.bat to create copies of each image resized to a maximum of 800x800 pixels.
To use this you will first need to install ImageMagick and make sure that its convert.exe tool is visible in the system path. Then copy the batch file below to your SendTo folder e.g. C:\Users\
Download batch file here: resize_images.bat