How to set memory size for video stream prefetch

[Admini]

You can use these command lines to limit disk cache size:
--disk-cache-size=XXX
--media-cache-size=XXX
--disk-cache-dir="D:\chrome_cache"

XXX is in bytes, and no more than 2147483647.(2GB)

If you mean RAM size used for video streaming, I think there is no way currently.