You should create a dir in \yoursite\blocks\youtube\templates\Mytemplate copy the view.php file that you can find in \yoursite\concrete\blocks\youtube to the templates dir you created above and edit customize the view.php to meet your size requirements.

Finally add the youtube block to the desired area in your web-page and choose the custom template that you created with the new youtube video dimensions.

Hope this helps !!

it means replace <?php echo $vWidth; ?> text & add a customized size?
ex: width:300px

Hi,
I tried what you said. It didn't work

I'd suggest using this add-on instead:
http://www.concrete5.org/marketplace/addons/youtube-pro/...
You just assign the width and it will automatically take care of the height.