I once asked this questions to visiting author Grath Nix, and he said that every book is as long as it's going to be.
Don't try to make it longer, because it won't be as good. If you want a longer book, develop your plot more. But don't add in things like plot twists just for length (unless of course you're doing NaNoWriMo). They won't work.
*Don't expect to see me around much in the next couple of weeks. School has started again, and it'll be a couple of weeks before I've settled in. If you've asked me for a critique, you will get it, but not for a little while. Sorry*
