Resize all images in current directory to 600px height:
for i in *; do mogrify -verbose -resize x600 "$i" ; done

Resize all images (*.jpeg, *.jpg, *.JPG) in current directory and subdirectories to 800px height:
find -type f -iregex '\(.*\.jpeg\)\|\(.*\.jpg\)\|\(.*\.JPG\)'|xargs mogrify -verbose -resize x800