Dumb Question: How do you insert a youtube video in a post.

Now people do this its VERY ANNOYING for sure.

No matter what I do, the video always comes out in this smaller size. 
When previewed, it's the proper size but when I "post" it, the screen
goes blank (all white) and I have to refresh it to get it to work, but again,
at this smaller size. Every darn time.

All the best,
Nonoise