I was very comfortable with the size of core books in 3e, thus for me ~300-350 pages of each of the 3 corebooks or equivalently ~900-1000 pages for the core as a whole is the sweet spot.
As for fluff, it's complicated... I love fluff but
core might be better off if conceived more as a toolbox. I don't think I really need
spell fluff or
character "building blocks" fluff, but OTOH I certainly need
monster fluff and perhaps even
magic items fluff.
It shouldn't go overboard of course. There is no need for habitat and ecology details for every single monster, especially since IMHO monsters are more interesting when they are more varied, and that includes not every monster having an ecology but instead some monsters having
unknown origin or even totally unrealistic or nonsensical backstories (like in tales). I understand that some DMs want a "sensible" world where every monster has a place in the food chain and evolution chart, but for other DMs that can be terribly boring.
Often I think that what really captures me, is the kind of information that lies at the blurred boundary between crunch and fluff, i.e. something that is
usable in the game but is not represented by numbers. For instance, description of monster
tactics, not just for the combat phase, but also in a more general sense, how the monster should be played by the DM while in the other phases of the game.
So in conclusion, I'd like the fluff of the
PHB to be mostly limited to races (but the current write-ups are more than enough for me), but I totally want the weapons & armors descriptions too. For the
DMG I'd like some fluff for magic items. For the
MM it's hard to tell, but enough fluff for each monster to be "droppable" into an encounter, adventure, campaign or fantasy setting with relative ease.
Also, they should not forget that the most effective "fluff" is the
artwork