Parameter substitution takes place on all properties of all objects that a form or report is constructed from. In principal, there is no reason why something like a form width cannot be parameterised.
However, at present it is effectively impossible to parameterise any property that is actually used in design mode (such as form width). If you really do want to play this trick, you should be able to do so, but you will need to hand-edit the XML form or report definition (and it will be lost if you make further changes via the form or report designer).
To be honest, we have not actually tried this, but it ought to work :)