Hi Paul,
You could define a class for example popupWidth900 and assign it to the forms you want to popup with a width of 900px. If you want a popup with a width of 1000px just duplicate the class in your css and rename it to popupWidth1000 and set the width.
In your css make sure this is only applied when the form is opened as a popup by preceding the class with the class for a popup (don't know the exact name, but use firebug for example)