The maximum YouTube video upload file size is 256 GB.
YouTube's current upload guidance pairs that file-size ceiling with a duration ceiling: a video may be no larger than 256 GB or longer than 12 hours, whichever is less. Thus, 256 GB is the correct answer when the question specifically asks for maximum file size, but it does not mean every 256 GB file will be accepted.
The practical limit also depends on account eligibility and technical conditions. Accounts normally must be verified to upload videos longer than 15 minutes, and YouTube recommends an up-to-date browser for files larger than 20 GB. Creators whose files exceed the cap are advised to compress them before uploading.
Older videos can exceed today's duration limit because YouTube has changed upload rules over time. That history sometimes creates confusion in discussions of the “largest” possible upload. The file-size answer remains 256 GB; the important neighboring qualification is that YouTube separately caps duration at 12 hours.