Finding Dimensions in Illustrator.
If a customer sends their artwork to scale, finding the dimensions is fast and easy.
Once Illustrator is open, click File < Open, and navigate to your file and hit Open. Once your file is open, hit Alt + 0 on PC or ⌘ + 0 on Mac to bring the artwork to front and center.
Hit Ctrl + A on PC or ⌘ + A to select all the art in your file. Look in the top bar or Transform window and you'll see the width and height of your selection.
Dimensions in Metric or Pixels?
If your dimensions are in Metric or Pixels you can easily change them to inches by going to File < Document Setup... and change your Units to Inches.
Often times we get weird files that are in masks, or locked layers, or somehow else not easily selectable. A great trick is to create a new layer
Now simply select the Rectangle Tool (M) and click and drag to draw a box over your image.
After drawing the box in the top bar or Transform Window you'll see the width and height of the box you drew which should be the correct dimensions.