photo-video question

[Bill Jemison]

Here is the situation. I have an .mp4 video file, 1920x1080 dimension opened in PS. The video was shot using a 21mm lens. There is an object in the video which has no recognizable object for comparison to get a perspective on size.

So, I want to add as an overlay an object of well known size (the full moon) in the proper size for the overlay. The reference image was shot at 24mm with the same camera at 5760x3840. I'm thinking the 3mm difference shouldn't matter much, but would I be correct in thinking that I should

1. resize the image (aspect constrained) to the dimension of the video. Since the video isn't the same aspect ratio as the overlay image, that requires being resized to something other than the video's 1920x1080
2. I chose to resize to 1620x1080 and then add 300px to the width to make it 1920x1080 to match the video and the full moon is still round.

At this point, can I reasonably add the file, as re-proportioned, as a fairly accurate comparison for the size of the subject of the video?

Thanks!

Bill Jemison