Size doesn't seem to matter

[Silver Surfer]

If I copy a selection having a specific size, say 5" by 6" (per the ruler on the Paint.net window) from one pdn file and then paste that selection into another pdn file it is no longer 5" by 8". Instead its dimensions are something like 1.25" by 1.5" per the ruler for the second file. The two pdn files are not the same height and width if that makes a difference. I really want the pasted selection to be the same size as the copied selection. What am I doing wrong?

[pdnnoob]

The dpi (dots per inch) of each image may be different. Check the resize window (ctrl+r)

[Silver Surfer]

The dpi (dots per inch) of each image may be different. Check the resize window (ctrl+r)

Bingo! The resolution pixels per inch were different. Made them the same and all is well. Thank you.

[Rick Brewster]

At one point I looked into what it would take to make sure that an image with (e.g.) DPI=120 would show up at the proper "physical" size when pasted into an image with a different DPI (e.g. 96).

As it turns out, DPI isn't something that is consistently written to the clipboard by other applications. To the end-user it would just randomly work or not, and it would suck.