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You basically want your DPI to be around 100. 24" 1080p means 92, 27" 1440p means 109. Double these numbers for HiDPI; that's why 27" is the sweet spot for 5K.

If your eyes are older, or you sit pretty far away from your monitor, you can have lower DPI (e.g. 27" at 1080p is 82dpi) without losing visual quality, only some screen real estate. Likewise, 25" 1440p is 117dpi and may be too small.
 
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