You can resize the stamp after it's placed. Just use one of the resize handles, click on it and change it's size.
If that is not sufficient, and you want a smaller stamp, you unfortunately have to start with a smaller stamp file. The size of the stamp image or graphic in the stamp file is the size it will have when you place it. You don't have to completely re-do your stamp: Just start out with a smaller stamp image, then copy and paste all form fields from the too large stamp to the now correctly sized stamp, assign the page template name and you should be done.
... or just use Replace Pages and slip the new, smaller artwork under the existing fields and reposition them. Then you don't need to change the page template name.