Check the avada-documentation:
https://theme-fusion.com/support/documentation/avada-documentation/ -> Menu -> Extra -> Videos In Lightbox -> How To Set Video Size in Fusion Theme Options
How To Set Video Size in Fusion Theme Options
Step 1 – Navigate to the Avada > Theme Options tab.
Step 2 – Go to the Lightbox tab, and locate the Slideshow Video Width and Height options. Set the width and height of your video by entering a pixel value into the corresponding setting. For example,1280px for the width and 720px for the height.