I wanted to make sure I got this right for you so I ran your question by one of our engineers. Here's what he told me:
Drawings created on an iPad Pro before the ability to choose a size would be 2732 x 2048 pixels. This corresponds to the “iPad Pro” format, either portrait or landscape, under “Digital Illustration."
To convert between pixels and dpi/ppi use “size in pixels = size in inches times desired pixels per inch” or “size in inches = size in pixels divided by desired pixels per inch”. So, for example, the iPad Pro size is 2732 x 2048 pixels which becomes about 9.1 by 6.8 inches at 300 pixels per inch. As another example, the “Large Postcard” size is 5 x 7 inches at 300 ppi so that is 1500 x 2100 pixels.
To see the format of an existing drawing, open it and then tap the gear icon on the top right of the header bar. This opens up a “Settings” menu and the top line shows the drawing format e.g. “iPad” or “iPad Pro."
I hope that helps.
Thanks so much for the detailed response. That makes sense. Just to confirm, I can assume that the resolution is 300 ppi/dpi and not 150?
If you use the size he mentions (9.1 x 6.8)... yes. Make sure you're using that iPad Pro format when choosing a document.