I am using raspberry pi zero w with success, however, it is oversized
and I can't see the whole screen. Is there any quick and dirty way to
reduce the display so it fits a 3.5 inch screen?

I’m sorry I don’t have the exact file name for you but there is a TXT file in /boot that contains the display parameters to adjust. Also, if you go to adafruit.com and look at any of their 5” or 7” hdmi displays there is a reference on the page to the same file with the parameters to adjust. I have a Pi-3 running WSJT-X and was able to adjust the parameters for a proper display on a WIMAXIT 10” hdmi.
Bill acj