If you use php on your site and share video from Youtube or vimeo, see how you can change their size easily, without changing every single post.
Let's say you have a post and it contains an embed code from Youtube. You will going to pass the post's content into a variable named $post.
Using the preg_replace function we'll change the size of the video.
This way will always keep the videos in one size, which is ideal if you have a lot of posts and you don't want to change them manually.
$post= preg_replace('/(width)=("[^"]")/i', 'width="600"', $post);
$post= preg_replace('/(height)=("[^"]")/i', 'height="400"', $post);