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First, per my books and the FAQs to the right of the forum, you should NEVER use photos larger than 1000x750 pixels in size
unless absolutely necessary. It nearly always chokes the program.
Then (also per my book), BEFORE you import the photo into your project, go to Edit/Preferences and uncheck the option to Scale to Frame Size so that your photo will not be comformed to the frame size. (If you've already imported the photo, you can right-click on it on the timeline and uncheck this option.)
Finally, when working with photos, press Enter to render your timeline whenever a red line appears above the clips. When the red lines turn green you'll see a much more accurate preview of your final, rendered video.
Lots more tips like this in my books, if you're interested!
Unless PrE has upped the max. pixel dimensions, the largest Still Image that you can Import will be 4096 x 4096 pixels. Even if the max. pixels for the Still Image is not exceeded, many of these will choke your computer and your Project.
It also sounds like you have Scale to Frame Size checked, and that will Scale your panorama, which is not what you want.
This ARTICLE will give you some background, and some tips. Note: the first part of that thread are from when SD Projects were the norm. The pixel x pixels sizes have been increased for HD Projects. Read down for those.
With very large pano Images, a better program would be AfterEffects, which handles much larger Still Images, and in a different way, than PrE, or PrPro.