I am having an issue with one artboard in my document, as you can see it is much larger than the rest, how do I fix this?
If it's your pasteboard having that issue:
Try going to Preferences > Guides & Pasteboard > Pasteboard Options and you can adjust it from there. If that still doesn't fix your issue then, as the other answer suggests, you more than likely have something hidden on your pasteboard that's causing the size difference. Try removing the item individually and your pasteboard should reduce.
If it's your individual page having that issue:
Make sure the top-left handle is selected (since your content from your image is all near the top-left and you're needing to bring the size in from the bottom-right) as seen in that image and from there you can adjust the width/height to fit your needs and make it the same size as the rest of your pages.
This also happened to me recently with a file I took over from other designers.
It might be you are importing an IDML saved from a newer version? In any case this probably happens because of a hidden small object thrown outside the artboard.
What i did to resolve this was copy everything on the page (dont use select all but actually select via mouse all the items on your page without selecting anything outside the page), delete the page completely, add a new page and paste your clipboard.
This way you are sure to move your visible artwork to a fresh new page without leftover hanging artwork.