Two Facebook surfaces, one studio
Posts in the feed
A collage you intend to attach as a photo post should be 1:1 or 16:9. Square (1080×1080 CSS, ~2160 exported) is the least surprising on mobile. 16:9 (1920×1080 CSS) matches the cinematic still people expect next to a link preview (Facebook’s link share still often behaves like 1.91:1 / 1200×630). Keep cell counts modest so the feed thumbnail is not a mosaic of unreadable faces. Export JPG unless you need PNG edges, then upload in the Facebook composer — this studio does not publish for you.
Covers are not a preset here
Desktop profile covers have long used a wide, short crop; mobile shows a different window. Current public guidance still orbits roughly 820×312 CSS for the desktop cover with a tighter mobile safe zone in the center. Collage Studio does not offer 820×312. Practical path: build a 16:9 board with the important faces in the vertical center, export, then use Crop Image to cut the cover band, or let Facebook’s cover editor crop while you watch both desktop and phone previews. Event headers (often cited around 1200×628) are closer to 16:9 — still not pixel-identical, still a crop-and-check job.
Page vs profile vs ads
Organic posts are a photo upload. Catalog and carousel ads are an Ads Manager problem: you design assets here, then assign cards and URLs there. Do not expect this tab to attach shop links to cells. For a Page cover, the same safe-zone warning applies as a profile. If the collage is a community recap, prefer 1:1 so groups and phones do not butcher a panorama.