There are no hard limits from our side regarding file size. The only restrictions are the storage capacity allowed on your Dropbox plan and the maximum upload size dictated by YouTube, which is currently at 128GB*.
YouTube also won’t accept video files longer that 12 hours*.
However, your plan must have sufficient monthly quota available in order for the upload to start. If the file is larger than your remaining quota in the current billing cycle, another attempt will be made in the next billing cycle or as soon as you upgrade your plan.
* Please note that YouTube may change these restrictions at will. Please refer to the YouTube support pages for official and up-to-date information.