The template is irrelevant. If your book is the wrong size it needs to be resized.
How far off is it?
Template is off by approx. 0.25. Not much. Can I reduce final PDF file size approx. 5% so nothing gets moved changed in the layout? I have a feeling I'm going to have to start over from scratch. Book is over 400 pages.
Are you submitting a PDF or an InDesign file?
If a PDF, you can just print it to PDF at 95%.
I'm submitting a PDF file to createspace.com. They have (apparently) very strict guidelines.
In that case, you can leave the InDesign file as is, and instead of exporting to PDF (which is normally recommended) you can print to PDF. In the print dialog, under the Setup tab, you have the option to scale the width and the height.
You can also, alternatively, export to PDF as usual, and then print the PDF to another PDF, and again, you will have the option then of reducing the width and the height.