Once you go over 120,000 words, printing costs climb significantly, and if you go lower than 80,000 then people will consider your work too short for the money. Most books fall somewhere between this bracket though many don't.
If you want to write a novel, fine, but don;t worry about this too much. I planned my first at twenty five chapters, expecting 80-90,000 words. I ended up with thirty five chapters and 114,000 words. My agent made me cut down to 100,000.
Just write it is my advice. Then worry about length.