Actual print size

Ha! This, in the comedy world, is called a Punchline Scramble!

🙂

Yes, and actually beyond the edge.

First of all, if you are printing this yourself, you need to know if your printer can actually print to the edge on that paper size. Some require a margin for technical reasons. Even if it it can, the following applies:

If you are printing this professionally, they usually print on larger paper then trim it down to size. For that to work successfully, you have to add "bleed" to the file, which is where you expand your background beyond the edges of the page. In InDesign, you add this dimension (typically 1/8" (9 points)) to your Page Setup.

This adds a red guideline beyond the edge of your page indicating how far you should expand your background elements.

Select the background gradient and resize it. 

However, the gradient could be locked, on a locked layer, or on the parent/master page. 

If the object is locked or on a locked layer, the Layers panel will show it can you can unlock it there.

If the object is on a parent page, go to the appropriate parent page and resize it.

What you see is a margin, likely the default. This is just an arbitrary inset the designer picks when creating a document. You are always free to ignore the margin. If you want to print to the edge you should include bleed. This act like a negative margin. When printing on stock larger than your trim size you would usually include crop marks (no need to draw them, they are an option in the print dialogue). Bleed allows object that print up to the ends of your design to extend beyond it to allow for some inaccuracies when cutting. 

If you are printing on stock the same size as your trim you need to use a printer than can print up to the edge of the page. This is uncommon as most printers need gripper, a bit of paper on the edges that can be used to feed the paper through the mechanism. If you are printing on stock larger than your trim just be sure to include bleed in Document Setup, extend any objects or images into the bleed, and print with trim marks. If you are sending files to a printer talk to them about how they want the document set up. Printers want people to set up their files properly to prevent problems, reprints, or delays. This means good print shops encourage asking relavent questions.