Can I Create a New Structured App that's a Copy of a Current One?

Here's the situation:  I am new to using Dita in Framemaker, but have become competent with the authoring process. I do not understand the development end of things very well, however. What I want to do is to have a second dita structured application that is identical to the first in every way except that in topic.template.fm, the page size will be set to 11x17, not 8.5x11.  And I want to be able to apply that structured application to some of my DITA documents, but not all of them.

I tried to create such a situation, first by copying and then renaming the DITA folder (I called the new one DITA-BIG) in Adobe FrameMaker 8\structure\xml\ and, within DITA-BIG, making that small change to topic.template.fm.

I then added the renamed structured app to structuredapps.fm (again, I did a copy and paste of the DITA-Topic-FM element already there), changing the name of the element to Dita-TopicBig-FM and the URLs to point to those in the DITA-BIG directory.

The new structured application appears in Framemaker when I select StructureTools --> Set Structured Application, and I can select/apply it to one of my documents. However, the document's page size doesn't change. I tried saving the document, closing and reopening the document, closing and reopening Framemaker. The new, larger page size set in topic.template.fm in the DITA-BIG structured app. is ignored.

If I change the page size in the original topic.template.fm file, the document that is supposedly using the new structured app changes size to correspond to the template in the old structured app.  But I need that original template to stay at its original 8.5x11 page size.

I am new to working with such files so I don't know if I missed something or if what I am attempting cannot be done.

Any advice?