What a network checks before it takes a file
Every network publishes a list of what it accepts, and every list is different in ways that only show up when a file is refused. The ceilings on size are the easy part: Discord takes 10 MB a file and 25 MB a message, Telegram 50 MB through a bot, Bluesky 100 MB, Instagram 300 MB, LinkedIn and Tumblr 500 MB, Threads 1 GB, TikTok 4 GB, Facebook 10 GB and YouTube 256 GB. The table above reads those figures from Preflight's capability sheet, the same one the composer and the API enforce, so the number here is the number a scheduled post is held to.
Length is the next cut. Bluesky stops at three minutes, Threads at five, TikTok and Tumblr at ten, Instagram at fifteen, LinkedIn at thirty, Facebook at four hours and YouTube at twelve; a Short is three minutes or less, and a reel on Facebook is 3 to 90 seconds. TikTok, Instagram and LinkedIn also refuse anything shorter than three seconds. Stories on Instagram and Facebook are a separate object with their own 3-to-60-second window and a 100 MB ceiling.
Containers, shapes and pixels
The format rules are where most refusals come from, because the file looks fine on a laptop. Bluesky and LinkedIn take only MP4; a MOV straight off an iPhone is refused. Threads and Instagram take MP4 or MOV, TikTok adds WebM. For pictures, Threads takes JPEG and PNG only, LinkedIn JPEG, PNG and GIF, Facebook adds BMP and TIFF — and none of those three takes WebP, which is what many export tools now write by default.
Shape matters on the networks that draw a feed. Instagram takes feed pictures from 4:5 portrait to 1.91:1 landscape and crops nothing else; it refuses the file. Threads takes pictures up to 10:1 and resizes anything outside 320 to 1440 pixels wide. TikTok wants every side of a video between 360 and 4,096 pixels. LinkedIn caps a picture at 36 million pixels, Telegram at a width plus height of 10,000 and a ratio of 20:1. A Facebook reel has to be vertical and at least 540×960 at 24 to 60 fps.
The browser can read a file's container, size, dimensions and duration, which covers every rule above except frame rate; Preflight reads that from the file after upload, with the rest of these checks, before a post is scheduled. The full table of rules per network is on the network limits page, and the character side of the same question is answered by the character counter.