I cannot find a good resolution with Duet Display on my 11" iPad Pro. No matter which one I choose, it’s letterboxed.
I updated to the very latest so the app “supports new iPad Pros”. Any ideas?
I was having that same challenge. I had to uninstall and reinstall DuetDisplay to get it to work. However, with the update to Mojave yesterday, Duet Display installed a new update today. I was able to change the resolution on the iPad display to a non-hidipi setting. All seems to fill the screen now. I have the new 12.9”, but it should be similar…hopefully…
I tried that, but no dice. There’s an option in the Duet MacOS app that says “best for 12.9” iPad", but there’s not an option for the 11"
Hmm, did you happen to update to 10.14.2? I could have the number wrong on the Mojave update. It treats the iPad as a regular display now.
Yes 
I updated MacOS and iOS yesterday and reinstalled both Duet apps.
I’ll try it with the other iPads in the house shortly and post back…maybe others will know more.
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I can’t get duet to work with the 13” iPad Pro either
OK. So, I was able to get full-screen with the new iPad Pro 12.9", the new iPad (6th Gen), and an iPad Air 2. Once I it synced from the Mac to the iPad, I opened up System Preferences/Display on the Mac. For the older devices were full-screen right away. The iPad Pro 12.9", I had to change the display setting to 2224X1668 (things look a little larger) on the iPad screen. It seems to stay once I reset it. I am attaching a screenshot, hopefully it will help. I didn’t try changing the settings in Duet itself.
Unfortunately, the resolution for 11" is nowhere to be found. I tweeted Duet and they said “this will be fixed shortly in an iOS update”. 
Good to hear they are on the fix.