Here's an even easier tip for finding the dimensions of your map. You can use the "Set Dimensions" option when you right click on the map after you load it. Then in Windows Explorer or Finder for mac, go to the properties, go to details, find dimensions, and you'll find a pixel dimension of your map. Keep that window open, go back to your map, place those dimensions in, the map will change. Then to make the page settings the same as the map, go to page settings, place those dimensions in the size, hit save, go back to your map and align it.
When you drag&drop, you can also just hold Ctrl+Alt when you let go of the map and it will not resize. Also useful for pictures and if you want to move things around or resize content, it prevents grid-locking.
2 years after you made this video, here I am, watching it and feel so grateful for your help! Thank you so much, this helps me a lot!! P.S. Love your voice, very soothing!! I wonder if you're a voice actor too. Must be a great voice to do narration!
Great tutorial! To add to your video, if you switch to Map layer before placing the map image you will not have to deal with resizing it (even with the grid still on), it'll just pop in there in its original size and roll20 will even automatically ask to resize the page