Interesting idea. As you know, you can do this now but you have to set the margins.
Your suggestion inspired us to make an example file that uses sliders to let you adjust the size while maintaining a square aspect ratio.
That file is demonstrated in this video …. https://youtu.be/_q6Ip2Rz6mg
This file has been posted on-line.
We will think about how we could also make a built-in option.