Keeping graphics frame constraint when resizing?

I have little square graphics that I would like to resize, whilst keeping it square. Is there a way or modifier key to achieve this?

I use the Properties Panel to set the exact dimensions I want.

Craig, yes, I did some experiments with the property panel, too.
I would have to do calculations, before I can make use it of this way.
I am looking for something, which lets me resize the graphics frame whilst keeping the ratio between x and y axis.
[edit] I see, what you mean, I have to open two of the locks - then type in two equal values to keep it square.