There are several ways to break up resource files.  Once common way is to
have a resource file per page, and then a central resource file which
contains shared resource elements.  Another way would be to break it up on
a per assembly basis (if that makes sense for your application).  I think
the main consideration though is that if you have resources which are
shared across pages, then you would want to have those located in a central
file.
Hope this helps.
Marin Millar [MSFT]