Adding videos to your site is free content, can promote trust, and keeps people on the page longer. Great.
You can just drop a YouTube URL into a Wordpress article and it will turn it into a video. Except you get these tiny, hunched squares that you can't put in the centre.
So here's what you do:
Go to the video's YouTube page, click Share, then Embed.
That will change what's in the box from the basic YouTube link to an iframe thingy. Copy that.
Make a new line in your Wordpress article where you want to put the video. Then change from the 'visual' view to the 'text' (i.e. html) view.
Pop the code in its place, go back to Visual, click the video - now you can centre it and it will be a decent size on your website.