You need 2 modules for this one.. Image Widget Crop and the Crop module itself. You can crop a image field with it. A lot of configurations... Look also at the documentation. See link..
After installation you see a menu link in 'Structure'.
You can set the aspect ratio and the minimum of size. Here i take a 1:1 ratio (=square) and i want the crop to be al least 300 px.
Afterwards you need to make image style.
Choose manual crop.
Take the crop style you want.
If the crop is bigger than 300 px i scale it to 300px.
now you make a content type with an image field. Set the form configuration to the widget.
Then you need to set the display also.
Choose the image style.
Finaly you can put a picture on the site. Ik took a large one and made a square crop.
In the display the crop is reduced to 300 X 300 px. Cool isn't.