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Can be done using your rectangle select tool.

Making sure when you do that these settings in your tool options dialog for the rectangle select are checked - 

*Rounded Corners

*Fixed - aspect ratio

Do your first Selection then go to Edit and from the drop down menu select stroke Selection. Making sure the colour you want the stroke is your forground colour  thats the one in the front. Stroke at 4 pixels - depending on the size you want your square. Larger - larger stroke size. While your selection is still active grow it  at 6 pixels or again more if your square is larger.

Stroke that selection 4 pixels. 

Make sure your background is transparent then go to image and crop to content That's it!

Export your finished square as a square(or whatever you want to call it).png Do not export as a jpg as it doen't support transparency and you will end up with a background of the colour of your current background colour