If you are trying to embed a Youtube video into your blog post, the best way to do it is pasting the Youtube URL directly inside your post. However, if you are using a Genesis theme by StudioPress, you might end up getting a tiny video.
This is a simple solution to adjust the width of all of your Youtube videos. Open your theme folder and edit the functions.php file, this can easily be done by navigating to your WordPress Dashboard -> Appearance -> Editor. Add the following code to this file:
// Embed width
if ( ! isset( $content_width ) ) $content_width = 900;
You can see that the video is a lot bigger now.
I set the width for embedded Youtube videos at 900 pixels, you can change to a value suitable for your content width. Hope it helps.