You would be better suited asking Wix Support first to see if you are actually able to change the minimum sizes when you use the Wix Editor settings to shrink the transparent video.
https://support.wix.com/en/about-wix/contacting-wix-support
Optional) Adjust the look of your Transparent Video by doing one of the following:
Resize the video : Drag the corners or sides of the video to adjust the size.
https://support.wix.com/en/article/about-transparent-videos-8722616
Other than making your own transparent videos and making them smaller than the 240 pixels, it might be that you can’t go any smaller.
See here for transparent video file types etc.
https://support.wix.com/en/article/supported-media-file-types-and-file-sizes