Ensure you're within the file limits for uploading content (Guide)
Video uploads are limited to 2 GB each (or 4 GB if you are a Pro member).
If you have a video that's larger than that size, the first thing to try is reducing the resolution or bitrate of the video, which can decrease the file size.
In many cases, you can do this through the "export" function of your video editor.
For example, when using's iMovie's Export to Quicktime function, you can choose the export resolution.
We recommend either 480P (854x480) or 720P (1280x720) resolution.
Or, you can use Handbrake, a free tool that can help you reduce your video bitrate.
Settings like these (or similar) will allow Handbrake to output a high-quality video that is a reasonable length.
Finally, you could also try breaking up long videos into a couple of installments.