How do I change the paper size for all my Invitations?

[Susang6]

I realized that many of my invitations are different sizes some are 5 x7 and many are other sizes and this makes the invitations different prices and would prefer that they were all alike.  However, when I go into edit I found that this is not an option.  So I have been changing the paper size manually...which is a long procedure due to having so many invitations.  Is there anyway to edit details to change the paper size ?   Thanks for your help.

[ColsCreations]

From product page:
Edit Details from upper menu -->
Product Merchandising from left menu -->
Change Options button
Select new options --> OK button

From back-end:
Actions -->
Edit Details .... same as above