I will address your first two questions.
You can measure the size of a digital documents one of two ways:
- In physical units + dpi. This is the case of the specs that you were given. They have requested a document that is 46in x 81in (physical units) at 300dpi (resolution).
- In pixels.
These two ways are equivalent. You can convert from one to the other by multiplying the physical units by the resolution (or dividing the pixel dimensions by the resolution).
So, in your case, you can create a document that is 46*300 x 81*300 pixels (13800 x 24300 pixels).
This is largely discussed in discussed in this other answer:
Will increasing the dpi of my image (without re-sampling) hurt print quality?
You don't need to do this calculations yourself in Photoshop, by the way. You can create the document directly using the physical + resolution dimensions. Just make sure you select the proper unit. Photoshop will create the document with the correct pixel dimensions.
In terms of what type of information you should ask in the future, I think the specs that you were given where perfect. Most of the time clients forget to give you the physical dimensions. They would say: I need a 300dpi image. In this case they gave you the dimensions and the dpi, which is perfect.
In the future, just make sure you have either the exact pixel dimensions or the physical + resolution combo as they gave you this time.