Presets vs exact pixels vs percentage
Presets are the social boxes: 1080×1440 (3:4 post), 1080×1080 (square), 1080×1350 (4:5), 1080×1920 (story). Exact pixels is for a portal that printed “max 1600 px on the long edge.” Percentage is for “make this 50% for email.”
Aspect lock fits the image inside the target box without stretching. A 4032×3024 (4:3) photo into 1080×1440 (3:4) will letterbox unless you crop elsewhere — we do not invent a crop. Turn lock off only if you intend to squash.
Compression in the same pass
A 1080×1440 JPEG at quality 80 is often 150–280 KB, which is what you want for a feed post. PNG stays PNG unless you compress to JPEG/WebP; transparency survives on PNG and is lost on JPEG. Re-encoding JPEG/WebP typically drops EXIF/GPS. That is usually what you want before posting a beach photo.