Yes, this is a possibility.
1. Fixed width / height
In the properties of your pop-up page you can set width / height, but these are not dynamic.
You can make a custom class (or .mx-window class) that overwrites the width / height:
If you implement this option, make sure to turn of “resizable”, this option will make resizing the popup window overwrite your values!
If you use this option make sure you adjust the top / left values as well otherwise you'd have an 80%-width popup that starts from half your page and ends far outside your browser window