In general I prefer more detailed backgrounds as I don’t improvise particularly well and prefer to have the answers to why the character has done something in the past provided for me. Also, If the answer to ‘how do you know that?’ is ‘it appears in an unrelated bullet-point on the character sheet’ then I would consider that to be an issue with the character sheet.
I understand that other players prefer a more freeform approach and will happily roleplay for hours off a few lines. There are some games in which I have done this and enjoyed it as it was appropriate to the game style but others in which I floundered through lack of background detail. A particular pet peeve is lack of character relationship information. Sure, if the character doesn’t know many other characters then that section will be minimal, but where the character is in a large group that has known each other for a while then all of the other members should at least be listed.
From a gamewriting view, I will tend to longer and more detailed character sheets because of my own player preferences. I would also tend to agree with Hannah that every word is something that has had to be created out of nothing. Even I’m not enough of a masochist to do this unnecessarily, so if I’ve written something, there’s a reason for it to be there! Something that looks like flavour text could include plot points that would not necessarily be apparent to the character at the start of the game, but could require interaction with others for their significance to become clear. It may well be more vital for another character’s plot than the character on which it is written.
For this reason I think that Zanni’s approach could be unhelpful as it eliminates these specifics and would tend to lead not only to that character having a poorer game, but others that interact with it also missing out. It also eliminates the possibility of reexamining the background in light of new information that arises in the game, so could potentially cramp character development.
That said, I would agree that a character sheet that doesn’t fit in your cleavage is too big physically (unless the game format involves a place to keep it). I would argue that this doesn’t particularly relate to the level of detail on the character sheet, but the formatting. As far as I can tell, the larpwriter output is particularly awful in this regard - it might look pretty as a pdf but is a pain to carry around at a larp. Formats involving booklets or other layouts are preferable - if they come down to one or two sheets of paper, fantastic!