By using the Step and Repeat Browser, you can create squares with rounded corners very easily. 

1. Draw a circle. 
2. Make a snapped selection of the circle using the Magic Wand Tool.
3. Go to Edit Menu > Step & Repeat Browser.
4. In the spacing options, enter 0.1cm.
   
   

   
   

5. Press the standard mirroring button and watch the rounded square be created:
   
   

If you want to set a specific size for this square, you can now select it using the Magic Wand Tool, and go to Edit Menu > Resize.