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iMac 5K Display Resolution Scaling
The 5120 X 2880 native resolution in a 27-inch size means that intermediate resolutions are eminently viable in terms of the size of text and user interface elements.
The 3200 X 1800 option delivers rather small text, but it will no doubt please many users for a massive working area and with even small text remaining very sharp and clear.
Tip: to see all the scaling choices, option-click on Scaled.