Oversized or wide sunglasses frames add horizontal width that counters a rectangle face's length, while narrow, small-lensed frames tend to stretch an already-long face further.
The Styling Principle
Introduce visual width and interrupt the vertical line — horizontal volume at the sides, fringe or bangs that shorten the forehead, and frames or hairlines with a strong horizontal emphasis all work against excess length rather than adding to it. Oversized or wide sunglasses frames add horizontal width that counters a rectangle face's length, while narrow, small-lensed frames tend to stretch an already-long face further.
Sizing
As with prescription frames, sunglasses width should roughly match your face's widest measurement — for a rectangle face that means sizing relative to forehead or jaw.