TL;DR - Using Pro version (in-app purchase) I can’t set a headless Mac ( Screen Sharing) to resolutions appropriate for viewing monitor. Strangely, this app allows me to set the resolution to a variety of choices, just none that fit the native, & common, resolution of the many 24” monitors I own (1920x1200). 1920 x 1080 is the closest available option.
The developer was quick to respond via Twitter, but said it couldn’t be done unless I actually plugged the monitor into the mac I want to change the resolution on or purchased some HDMI hardware dongle. Not true as I found two other apps that actually work. One was really expensive & required installation as a startup service, etc. The other is $3 & is a cleaner interface & even allows me to set my 5K iMac to run at the full 5K.