Imported PNGs are huge on the canvas

Steve,

As a curiosity, 16383 px x 16383 px is the size of the entire (normal) Workspace, also known as Canvas (taken from PS speak).

Sometimes, (certain) things may fail or stop working for no apparent reason.

The following is a general list of things you may try when:

A) The issue is not in a specific file,
B) You have a printer correctly installed, connected, and turned on if it is physical printer. Or you may use Adobe PDF/Acrobat Distiller as the default printer instead.
C) It is not caused by issues with opening a file from external media.
D) It is not caused by other applications (you may close down/disable everything else running, including browsers and antivirus, then try again). There are quite a few usual suspects that may disturb and confuse Illy (job description Adobe Illustrator), and they can make her move in mysterious ways (even more than usual, some would say).

You may have tried/done some of them already; 1) and 2) and 3) are the easy ones for temporary strangenesses, 4), 5) and 6), are specifically aimed at possibly corrupt preferences.

If possible/applicable, you should save current artwork first, of course.

1) Close down Illy and open again;
2) Restart the computer (you may need to do it at least 5 times);
3) Log out of your Adobe account and log back in;

4) Close down Illy and press Ctrl+Alt+Shift/Cmd+Option+Shift during startup (easy but irreversible)

5) Follow the instructions here, you can preserve the preferences as described:
https://helpx.adobe.com/illustrator/using/setting-preferences.html