Using percentage:
$ mogrify -resize 10% *.png
Using dimensions:
$ mogrify -resize 120x120 *.png
Lots of other alternatives from ImageMagick's documentation:
-resize scale% -resize scale-x%xscale-y% -resize width -resize xheight -resize widthxheight -resize widthxheight^ -resize widthxheight! -resize widthxheight> -resize widthxheight< -resize area@
Fix image aspect ratios for Instagram:
$ mogrify -resize 1080x1350 -gravity center -extent 1080 *.jpg