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 not such a 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 choose 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.
You can see the header video at https://video-background-store.myshopify.com/collections/frontpage/products/adidas-classic-backpack-legend-ink-multicolour
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.
You can see the featured video in action at https://video-background-store.myshopify.com
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.
You can see a sample background video configuration at https://video-background-store.myshopify.com/pages/full-page-background