Yes, I would need the whole photo in there, which is why I was having difficulty.
But after posting my inquiry, I ate some chocolate covered esspresso beans and gave it another try and I'm pretty sure that:
1. I'm an idiot, and
2. I found the answer!
I opened the photo, in this case a horizontal photo, and resized it so that the width was 1920, and maintained aspect ratio. THEN, I resized the canvas by absolute size to 1920x1920. Perfect.
DUH, DUH, DUH AND DUH. So simple!
Sorry to waste the forum space for this, but maybe someday some other person will have difficulty with this and here the answer will be!