How can I make a button the same height as it's width

How can I make a button the same height as it's width

  

I am a new developer, and am trying OutSystems to see if it's possible to create apps with it. I have created a mobile application, divided it into two columns, put buttons into the columns, and set them to (fill parent).  I would like to have the height attribute set to be equal to the width.  That way both will adapt if the phone goes into landscape, etc.

You can use CSS and apply percentages to the widths and heights... have you tried it?