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Are you on a multi-monitor Mac? see: Photoshop CC 19.1.1: Images opening unexpectedly small (macOS) | Photoshop Family Customer Community
There's a workaround in the thread above.
I see Tanuj Sandal just marked this workaround as correct answer. However that might be helpful I think there's something interesting why this bug may happen still for multi-monitor. The bug was fixed partially so for the case of relocated 'Tools Bar'. Later I found it happens also when 'Options Bar' is moved to other place than usual. I don't use multi-monitor, but it may affect wrong behaviour (even if on one monitor 'Options Bar' is set in right place). Read my last post:
You may. be looking for this if your Retina screen is displaying the images too small for you.
Finder > Applications > Photoshop CC2018
Right click on Photoshop CC2018 to bring up Info and checkmark "Open in Low Resolution"
(Note that this applies only to Photoshop CC2018 images. If this does not solve the problem, return to Info and remove the checkmark.)