@tedpenner -

If you hold the Shift key down while you click and drag one of the corner scaling points on a shape then it will preserve the horizontal/vertical aspect ratio.

Kiron

Thank you very much!  That helps.

So now the question becomes how to create a shape of a particular size aspect ratio to begin with and drop it into miro.com with a transparent background.

Step 1: Make your shape to scale using https://gliffy.com

Warning that this company requires a subscription and an email to cancel it which deletes all of your work.

Step 2. Make a shape transparent using https://remove.bg 

Step 3. Put it into my whiteboard at https://miro.com

Step 4. Shift + drag the corner in miro to make it bigger without loosing the aspect ratio.