Small view in library and develop mode. How do I get back normal format ?

"I work in LR 5 and 6 for a long time.  Suddenly my view of a photo is about half the size than usual although my navigatorsetting is at fitt or full.  I only get my normal size on 2:1 ! ... It happened on the start of LR5 on my IMac Mojave on a collection of photo's size 1280x850 wich always gave me a view of about 80% of the viewing area in both librarie and develop mode."

That's expected behavior for LR 5 -- Loupe Fit and Fill won't enlarge the photo larger than 1:1. See this feature-request thread of people complaining about it in 2014:

https://feedback-readonly.photoshop.com/conversations/lightroom-classic/lightroom-hidpiretina-support-doesnt-work-well-with-45k-screens-allow-fit-view-to-stretch-small-photos/5f5f456b4b561a3d424af02b 

I just tested my LR 5 to verify that's how it behaves. (I don't have LR 6 installed.)  In a later version Adobe changed the behavior, not sure when.

Your screenshot is consistent with the LR 5 behavior,  showing a 1280 x 850 image displayed on a 5006 x 2672 screen.

The image shown in the screenshot is a JPG. Is it possible that you've resized the original and imported back into LrC? You can you check the pixel dimensions in Metadata panel. 

For that image, what does LrC report as resolution?

Perhaps I am seeing the screenshot incorrectly, but you are in the  Library Module?

So, what happens if you delete the library previews?

hmm, but it also fails in Develop Module. Different preview.

Does your iMac have a retina screen? It's likely a compatibility bug between the very old LR5 and retina screens on Mojave. If so it will never get fixed as LR5 hasn't been supported for a very long time.

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