Make a cube the size of the green square, add a boolean modifier to your mesh and use the cube as the target object, set it to intersect.
All this is done from the top view, 7 on the numpad
This represents your imported .obj mesh:
Add a cube:
Scale the cube to the size you need. Hit S to scale, then hit X or Y to scale on the X or Y axis... Try it out.
Next add the modifier to your base mesh, in your case you need the Boolean modifier. Change it to "Intersect" then click the little eyedropper and select the cube as the "Object"
Then hide the cube and your base mesh should have been trimmed into the shape of your green square.
Appologies for the poor quality of the gifs, each gif can only be 2Mb max... :)
You can "apply" the modifier so it is no longer modifiable if you like.
Please click the tick next to this answer if it works for you so others can find this information. If you need more help, just ask! :)