I've never run into any limits myself, but I've given it a quick google:
The maximum file size for non-animated images (think JPG, PNG, etc) is 20MB. PNG files over 5MB will be converted to JPEGs. The maximum file size for animated images (like GIFs) and video is 200MB. Video uploads cannot be longer than 60 seconds. GIF and video files over 2MB will be converted to GIFVs
Non-animated images over 1MB for anonymous uploads and 5MB for account holders will be lossily compressed. This compression reduces filesize while maintaining quality. Large GIFs will be converted to GIFV, which also shrinks file size while maintaining a high quality, perfect for our expert GIF makers.
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