Keep them as short as possible. Do what you can to optimize them to the screen size of the device. We know that resizing images can be very costly on the CPU - video is the same.
Adaptive bitrate streaming is great for serving videos as it does accommodate for the screen size, and the available network - serving the 'best' quality stream it can to the player. But there are many tricks to streaming - making sure that the bitrates you are serving are the proper ones for the devices/networks your customers are using. In general - start with a lower quality bitrate to lower the startup time.
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