I've just started to do some video editing with Blender (Linux) and I can't tell what function it has and how it affect the final quality.
I've searched about it but I didn't find any explanation behind the value it should take. So:
1) What is the function of buffer size?
2) What value should it have?
According to the blender documentation found here
Rate The bitrate control also includes a Minimum and a Maximum.
Buffer The decoder bitstream buffer size.
the buffer size is related to streaming video. The link in the manual takes you to this article which describes what the buffer is. It sounds like you are editing for web streaming in which case your buffer size should be based on your target internet connection and the quality of the video you are delivering.
If you are referring to a different buffer setting in blender please elaborate.
To add an answer to part1 of the question, the reason for telling the video encoder how much data the player will buffer is so it can constrain its VBR rate-control to not underflow the player's buffer (video pauses and user sees "buffering") in complex portions of the video where it wants to spend a lot of bits.
Within the limits of the VBV buffer size, the encoder can use a really high bitrate for a few seconds, as long as the bitrate before and after is low enough for the player to catch up.
Without VBV-aware rate control, the encoder could hit the target file size, but have a bitrate higher than the target streaming bandwidth for an extended period of time.
