Answer:
click on the "Crop" icon in the tool palette
at the top, in the "width" type "140px" & in the "height" type "140px"
make sure there's nothing in the "resolution" unless you know for sure what it needs to be. If its for the internet, its probably 72.
then, click on the top left of your picture, and drag down to the bottom right.
then press enter.