This is a bunch of nonsense. First off lets differentiate between background and backstory. Background is a choice you make which comes with features and drawbacks and the DM will determine what backgrounds are allowed. Generally if you take a background you get the things that come with it, but nothing more. Having the noble background for example means you have two skills, a gaming set proficiency, a language and the position of privilige feature. It doe not mean youu are allowed in the castle or can ask for an audience with the king or can get money from the treasury.
The characters work with the DM and develop a backstory to fit the DM's campaign. This can be narrow in focus or wide open, but regardless it is the DMA and player that develop it. Most of the time it is the player who says what he wants, and the DM tweaks it to fit the campaign.
As far as length it can be short or long. As a player I prefer long backstories, but that is because much of the fun is making the character. I know one DM I know demands 2 pages of backstory for any character in her campaign. As a DM it does not make much difference to me, long or short.
The biggest thing to keeo in mind though is it is a BACKstory. It is what happened before the character started playing, and should have very little on the story going forward.