What this size is for
250 × 250 is a small square for avatars and thumbnails. The lock keeps a portrait or landscape photo from being stretched into that square.
Use it when a profile or list view only shows a small picture. The downloaded image fits inside 250 × 250 unless you turn the aspect lock off.
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.