With this tool you can resize the image to shrink or expand by giving new width and new height properties. Width and height properties can by pixel or percent value. 'Keep aspect ratio' checkbox allows to keep image size ratio width:height when you change any property.
Here are few recommended sizes of popular social media images.
WhatsApp profile image - 500 x 500 px
WhatsApp share image - 800 x 800 px
Youtube thumbnail image - resolution of 1280 x 720 (with minimum width of 640 pixels)
Instagram photo thumbnail - 161 x 161 px
Instagram Stories - 1,080 x 1,920 px
Instagram feed - 1,080 x 1,350 px
Facebook Stories - 1,080 x 1,920 px
Facebook feed For images with a link - 1,080 x 1,350 px