I just got a new computer with a high-resolution display and AstroImageJ is tiny. I have gone to properties and have tried running the program with the "Disable display scaling on high DPI settings" both checked and unchecked. It is set up to run 64-bit and is using the latest version of Java.
I tried downloading ImageJ to see if that was any different and set it up with the same version of Java and everything and it opens at a reasonable size. Below is a link to a screenshot of ImageJ and AstroImageJ on my desktop. Thanks!
https://www.dropbox.com/s/3ma1lsk3ffa0w91/screenshot.png?dl=0