Dragging layer into new file changes its size
Let's say I have a file that's 4'x12' and represents a space on an actual wall. I want to open certain files (of different sizes) and drag each file into the large file in order to arrange the images correctly on the computer before physically hanging the framed finals.
When I drag a new image into the larger file, the new image's size and the size of its copy don't match. That is, Say I drag an 8x10 into the large file, well on the large file (trying to control size with guides and rulers) the new image is some other size, but not the right one.
What am I doing wrong? How do I make sure that when I click and drag the new image that it will be properly represented?
What I think is the clue to your problem is the variable relationship between pixels and physical size in Photoshop -- also known as the resolution
Each file has a set resolution: the amount of distance in inches corresponds with a set amount of pixels. If you made a new document in Photohsop, this ratio is most probably either 72 pixels per inch or 300.
When you drag another layer into your large file, Photoshop will faithfully copy the actual pixels of the original into your larger file, with no regard for the original's relation between number of pixels and actual physical distance.
In you smaller files, the resolution may ver well differ. If a given file says it's 8" by 10", but it has only 72 pixels per inch, it will show as way smaller when you drag it your large file if that larger file has 300 pixels per inch.
To second Matt_2.0's comment, Photoshop is not the best software to do this with. Try either Illustrator or InDesign these programs have way better tools to adjust placed files' size. And work with a scal of, say, 1:10.