Update (06/03/2019): The tool now includes an option to select the page orientation (ie, Portrait or Landscape) in addition to size.

ALSO, there's now a free demo version of this tool available, which will only convert one page in the file, to either A3 or A4.

Update (23/09/2018): Now there's a version of this tool available that allows you to scale PDF pages in a non-proportional way. It can be purchased here.

Update (02/07/2015): Now there's a new feature to this tool that lets you scale to one of several standard page sizes: A4, A3, A2, Letter, Ledger or Tabloid.And if you need any other standard size let me know and I'll add it!
The scaling to these sizes preserves the original proportions of the pages, so it adds white space around them instead of distorting them.
The dialog for this new feature looks like this:

The tool's standard dialog

Using this new tool you can scale your PDF pages to a new size of your choice, while maintaining the original aspect ratio of the pages, as well as any objects located in them, such as form fields, links or annotations (including stamps).

The script allows you to select which pages to scale, and offers two scaling options:

• To a certain percentage of the original size
• To a fixed size, specified by you, using one of the following units:

• Points
• Inches
• Millimeters

When you run the script (from the Scale Pages button it adds to the Tools menu on the right) it looks like this:

(click to enlarge)
In this option we select to scale the current page to 50% of the original size. The result can be seen below (click all images to enlarge them). Notice the zoom is the same as before, and the actual page size can be seen on the bottom-left corner of the window.

(click to enlarge)
Alternatively, we can also scale the pages in the file to a fixed size, 100 mm in width in this instance:

(click to enlarge)
And the result (again in the same zoom factor):

(click to enlarge) Using this file we can also see that the meta-objects on the page have been scaled down. This is the same page magnified to 250% zoom, allowing us to see the scaled link (bottom left corner), form field (top left corner, called "Button1"), and Square annotation (bottom right corner):

(click to enlarge)
This handy tool can be yours for just $60 per license! You can purchase it directly via PayPal using the "Buy Now" button at the top of this post.