Hi Dennis,
Great it sounds like you're planning to upload something at some point. And yep everyone is challenged in the video area until they've done it a few times.
Length: it really doesn't matter and it depends what you're trying to get advice on. It could be as short as 30 seconds if that's enough to get your point across. I'd suggest somewhere between a minute and five minutes depending on what you're showing and whether you're playing and actual song where you want advice on the whole thing. You can show whatever you want, the technique you're having trouble with for example; it doesn't have to be a song, and if you're playing a song, it doesn't have to be one that's an official TG lesson either.
In truth the site could really use a more obvious way to let people know how they upload videos; where, technical specifics. But this is the link to the uploader for student reviews. Once you land there, you'll see a bunch of fields you need to fill in and then a place that allows you to choose your video from your computer's filing system. At step 3 there is info on the file formats it accepts; mp4, mov, avi, divx, mpeg. There's a size limit too in GB; 2.5GB, but you shouldn't have an issue there unless you record in HD and/or it's a really long video.
http://www.totallyguitars.com/index.php ... Itemid=223
There's a busking video uploader just for fun too, for when you're just sharing and not looking for advice. But I presume you're after advice at this point.
Just be aware that if you do upload a video for review by Neil or Vanessa, there's no guarantee that they will provide advice. But, you're sure to get comments from the community and I'm sure that those suggestions will also be helpful.
Let me know if you need more details on anything.
Have fun. Remember if you record and you don't like it, you can start again until you're happy with what you're putting up. Recording will affect your playing. It's just what happens.
Good luck
Michele