The size and aspect ratio of the widget can be customized in Widget design → Settings.

Regardless of the actual size of the videos assigned to the widget, this is going to determine how the component is going to appear on your site.

For video carousels, you can go to Widget design → Advaced Settings → Layout -> Change to 1 (this will make the video appear bigger).

For Single Video widgets, you can go to Widget design → Thumbnails → Thumbnail format and change to a fixed size; this way you can adjust the size of the widget any way you need.

This option is only available for embedded widgets.