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First you have to have a Steemit account, which it appears you do. Go to DTube.video and you'll be able to create & send a video. Upload a video you've created while on that site. Wishing you all the best in that endeavor.

Very well, but how long/big can the video be?

I haven't seen a limit on the length of a video. However, you'll get maximum views from a video that runs < 30 minutes. Doing a 4 hour video (hypothetically) wouldn't get as many views. I've learned this painfully as I tend to want to create longer videos to give more value. However, most people, including myself when I'm consuming video, want massive benefit and impact in a relatively short time. Something less than 10 minutes can often grab more attention.

What do you think @xerxes86? Thank you for your comment and stopping by.

Tried to upload a few videos, seems that only videos no longer than 10 minutes will be accepted.

That seems strange as the one you saw I did was longer than 10 minutes. They might have changed it. I'd recommend a quick Steemchat note to @heimindanger for clarification. Thank you for letting us know.

I don't clarify anything. Been down that way too many times in my life to know it's not even close to being worth the effort. Which seems to be the case with steemit and and dtube, not worth the effort. If all I get is total BS for my time and effort, then no, it's not worth it at all. Absolutely prejudged, that's the I roll, I have dealt plenty of web sites, and they are all about the same when comes to "clarification.