Photoshop / Calculation Question

Dunno if anyone can impart some wisdom.

By no means a photographer so out of our depth but over the years we have collected a few digital photos the Mrs want made into wall art.
She is actually quite good with Photoshop (well better than me) and wanted to knock up a mock image of what it would look like on our wall.

Someone suggested taking a picture of the wall with a sheet of A4 on it and then using that to calculate what the image sizes she wants will look like on our wall so she can play around with designs e.g. multiple images

Anyway she must be fucking something up cos the sizes dont look right. How do you pros do something like that. I have seen specialist software but no need when its a one off. How would you calculate said scenario.

Thanks for any advice. She is driving my nuts. :crushed: