Pick a distance and a sharpness target — see every resolution and size at a glance.
PPD (pixels per degree) — how many pixels fit into one degree of your visual field. It combines pixel density and viewing distance: the same screen gets sharper (higher PPD) as you move back.
Recommended targets: 60 PPD matches 20/20 vision (1 pixel per arcminute — the usual minimum), 80–120 looks noticeably crisper, and beyond ~120 the eye can no longer tell the difference. Apple's “Retina” threshold sits at ≈57.
Sources: Visual acuity (Wikipedia) · Retina display (Wikipedia)