What this size is for
512 × 512 is a common square for app icons, profile crops, and compact product images.
It is larger than a 250-pixel thumbnail, so a face or product stays clearer, and smaller than a 1080-pixel social frame. With the lock on, a wide photo stays wide and fits inside the square.
Check the resulting dimensions
The resize controls show the original size and the size you will actually get. That second number is the one used in the downloaded filename.