There's a quote attributed to Abraham Lincoln where he claims that a man's legs should be "long enough to reach the ground." And that's what I expect from my RPG books. The specific length or measurement doesn't mean much to me. Whether or not it "reaches the ground" (is a complete and useful text) is the key measure.
On the short side, I am an appreciator of Fate Accelerated. While a number of companies have offered "quickstart" or "lite" versions of their games to appeal to newbies, they typically feel stripped down and incomplete. Fate Accelerated is a lighter expression of the Fate mechanics, but it feels like a complete game. I actually bought several copies at the last DunDraCon specifically to give to friends because it felt like giving someone a complete gamerather than a teaser product or orphan supplement.
For longer works, I think my favorite would be The Burning Wheel. The books are not only thick, but very dense with text and ideas. Every word is there for a purpose.
On works that aren't quite long enough, regardless of actual length, I've already mentioned quickstarts. They might be useful as a player reference, but not much more than that. The longest book I can think of that still didn't do justice to its material would have to be Oz: Dark and Terrible. It left out mechanics for magic in a magical setting and lacked a solid GM chapter with advice on putting all of the pieces together.
Speaking for my own works, Adventures in Oz is the longest thing I've ever written. I find mechanics very engaging, with other details being less important. So writing up the land of Oz setting material was the hardest work, but I knew that the game would not be complete without it.