The YouTube video embed is responsive so it should expand to the width of its parent. If you do need to set a width you can apply the Width CSS property through the Style Builder.
The YouTube video embed element is responsive so it will always fill its parent element.
If you need to fix the width or give it a maximum or minimum width you can apply a style to it.
This is possible because the YouTube video embed element has a wrapping <div> which is what the style will be applied to.