Hi, I had the same problem but this worked for me:
click “Tools” at the top of the page → enable “Toolbar” → click video → edit width and height info in the toolbar.
This is a fragile solution, you will need to experiment a bit. (Wix will try to keep your video box at the minimum of 240px, but you can trick her.)
For example I wanted a 160*290px vid, but the edges were cropped, not fitted. I had to increase the size by hand just a tiny bit and then decrease back. It’s good now, but I’ll never touch that video again.