What is the best size for my video?
Video Background adapts the background video responsively to the section where the video is displayed, so there is no such thing as the best size for the video.
However, it is important to consider the target of the video on your Shopify store when it comes to choosing the dimensions and more specifically the aspect ratio, i.e., the width/ height ratio.
For example, header sections are in general full-width and short such as the screenshot below so it is preferable to choose a video that is wide and short.
However, for other more squared sections such as image sliders or featured images, it will fit better a video with a small aspect ratio, such as the standard 16:9.
The screenshot below shows a background video that would fit an image slider or a featured section on the home page.
It is also possible to put a background video behind the whole page. In this case, a standard 16:9 video will fit better most screen sizes.
As the videos will adapt to the section where it is being displayed, on mobile view, since it is a lot more vertical, depending on the video you setup(either Section Video Background or Element Replacement Video) you may notice that on mobile, the video overflows the screens, perhaps cutting it off. This is expected IF the dimensions of the video and the section, where you're placing the video, are very different on mobile.
However, this can be solved in a few ways:
1. By changing the size of the section on mobile
2. By uploading and configuring a different video in different dimensions for mobile ( in similar dimensions of the section on mobile)
3. By configuring a responsive video only on mobile and in that case you will just have to remove the section on mobile, because the video will be placed in its own container and not as the replacement of the image