Before running this trick to get a perfectly scaled res on your iPad Pro and you are running Mac OS X 10.11, you'll need to temporarily disable System Integrity Protection (SIP) by booting your Mac into Restore mode (Cmd-R) and run 'csrutil disable' from the Terminal.
Anyways, here is the trick.
Download SwitchResX, install the preference pane. Launch the SwitchResX preference pane and create a scaled resolution of 1366 x 1024. This is exactly half the screen res of the iPad pro's retina display which ends up creating a very clear pixels at fullscreen. Only create a scaled res, otherwise you could wind up creating a screen refresh that is incorrect for your Mac's display. You can also try 2048 x 1536 which is an average between the half res and fun res of the iPad display, but to me it is too squinty. You can go a step further and the scaled res to 2732 x 2048 but now we're talking really tiny. But you have these three options to choose from. SwitchResX allows you to create 2 free scaled resolutions to try before buying the software for $15.
It would be great in the future if AstroPad Desktop could add these scaled resolutions for those who like to use AstroPad in Fullscreen mode. But until then, SwitchResX can do the job.
If you are running a RAID0 on El Capitan Mac OS X 10.11, you'll have to backup and redo your system and install 10.11 on a non-raid disk. When I redid my system (which was RAID0 with twin SSDs), I had to copy the Mac OS X installer to my desktop, then it would allow it to install the restore partition which is needed to turn off System Integrity Protection. You can turn SIP back on after you've added your two favorite screen resolutions.
If you do not want to make a custom res, there are two 4:3 resolutions built into SwitchResX that may fit the bill. One is 1600x1200 which is a little larger than the iPad Pro's screen at 100% and the other is 1344x1008 which is a little smaller than the iPad Pro's screen at 100%. 100% is considered to be 1366 x 1024. It is retina display's actual resolution divided by 2.
You can also try using Apple's Display preference pane and while holding down the option key, select scaled and you should get 1600x1200 and 1344x1008 two scaled resolutions mentioned out of the box, but for minimum blurriness, 1366x1024 will be optimum and will require SwitchResX scaled res help.
If you somehow manage to get a blank screen because you did not use a scaled res in SwitchResX, you can reboot your system into safe mode by holding down the shift key and you can set your screen to a different its default resolution and reboot. If this does happen, you may have to repeat it twice.
Hope this helps someone. I love details and having a nice full screen that is not distorted or blurred is simply adds to the awesome goodness that AstroPad provides. Not that AstroPad at similar res' are bad. 1366 x 1024 is just clearer. Feel free to experiment with other resolutions based on the iPad Pro's pixel res. maybe a more HiDPI res for those who have really bad eyes!
This info is based on simple mathematics. Basically getting the pixels to fit in a perfect ratio where the dots are 1:1 and remain square on your iPad Pro.