Generally, around 100,000 words is a good length. But I hate to hear when people (whether they be other writers, agents, etc.) that a book HAS to be a certain length and there are no exceptions. And I'm not so naive to think that trying to sell a massive ms is either easy or very possible, especially for someone like myself, who has not established a name in the literary community as of yet. That does not mean I'm going to dumb down a story I want to tell that I believe has to be told a certain way. Believe me, I've considered paring some of my manuscripts down, but each time I think about it, I realize that if I did so, I might as well throw the entire ms in the garbage, because it would lose far too much.
I'll transplant something here that I posted from a previous forum...
A few of my completed manuscripts run well over that, as you might see in my signature (that isn't a typo). And it's not that I've filled those books with endless pages of needless descriptions. On the contrary, they deal with dozens of characters over a long period of time and events, so the length, in the end, is justified. I've divided them into separate "acts", which are in and of themselves connected as a single story, but could just as easily be printed as individual books. The first book in that book series, my 3rd ms, is three acts, the second book (4th ms) is four acts, the third book (5th ms) is two acts, so I have a particularly interesting task ahead of me in trying to sell each as a part of a much larger series (it spans 5 books, two of which aren't written yet, made up of 13 total acts) which will probably become 13 individual books if and when they are published.
This isn't to say every ms I write from here on out will be as long as these (in fact they will not be, I've already planned half a dozen new projects and the stories don't call for that), but if it is necessary to get a massive tale told, then there should not be a reason to fear doing what needs to be done. And if you think otherwise, ask the ghosts of Chekov, Dostoevsky, Flaubert, Proust, Faulkner, Mann, and Nabokov if they thought War and Peace or Anna Karenina were too long (they would damn you to hell for saying so). I am not comparing myself to Tolstoy, please don't misunderstand me. I'm merely pointing out that even incredibly long books can work, so long as they're well written.
Most people will still call me insane or simply have a hard time believing that what I've done truly is necessary. Until you've seen it yourself, you cannot know. Just like so many people have probably already said, a book should be as long as it needs to be.
PS...I write on MS Works, 12-point font, Times New Roman, single-spaced, 1-inch margins