+1 Similar issue but different canvas size.
Software version:
Paint.net 4.2.2
Office365 Pro Plus 1908 (build 11929.20300)
Steps taken:
Take a screenshot that is above canvas problem size. My canvas problem is 817 x 864. so I take a full-screen screenshot.
Paste said screenshot onto Outlook body (HTML format) NOTE: not an attachment.
Send it to myself or anyone.
Select the image and copy/paste onto Paint.net
Create new tab/canvas. Paint.net detected image size at 817x864.
Pasting the same image onto Word, msPaint, and other application results in original image size.
I have test with both gmail and outlook (microsoft not office365) and get the same result.
orignal:
https://i.imgur.com/HigDnob.jpg
After outlook shenanigan:
https://i.imgur.com/Xv9aoZf.jpg
Paste in Word. it kind of resize but that's a feature in word.. i think.
https://i.imgur.com/chUOSSt.jpg