Can paper size be defined on condition in SAPscript?

The only place to define a paper size is at the form header. I have heard rumor of a way to override it dynamically at run-time but have not seen it in practice. That being said, there are ways to ease the pain of dual maintenance. Here is a simple approach that I used a few years ago for a client with offices in the U.S. and Ireland; each location required the same form in different sizes (letter size and A4, respectively):

1.) Create the letter-size form, let's say: Z_DELNOTE.

2.) Try to make the first form flexible enough so that it can serve as a 'master form', with the only required difference being paper size.

3.) Copy that form into a new one, Z_DELNOTE_A4 and convert the paper size to A4. Adjust any windows as needed.

4.) Set up output config to point to the first form.

5.) In your print program, insert logic before the OPEN_FORM to append '_A4' to the form name field when appropriate (e.g. based on plant).

6.) Perform all subsequent changes to the first form and overlay the second one as in step 3.

I hope that answers your question.