8. Ambient Wisdom or Myth: I wish if I knew if I were doing the right thing, rewriting my 300K novel into a trilogy, because the ambient wisdom is a first-timer can't sell a 300K book.
Some publishers such as Tor have been splitting authors' books into two volumes, which I think might have been the blip that ignited this questionable wisdom. Peter Watts was the first author whom I heard had this done to his book, but I've also seen other authors, such as Holly Lisle, successfully fight the split and keep their longer books intact. Rick Kleffel wrote an excellent column about the practice, btw, which you can read here.
In publishing, the hard and fast rule is that there is no hard and fast rule. Two recent first-timers' hefty debut novels: Susanna Clark's Jonathan Strange & Mr. Norrell (800 pages) and Elizabeth Kostava's The Historian (642 pages.) Other authors who routinely write big fat novels and do quite well that I can think of: Neal Stephenson, Tad Williams, Diana Galbaldon, J.K. Rowling and Laurell K. Hamilton.
Practically speaking, shrinking shelf space, the new trend for the oversized paperback, and rising production and shipping costs would seem to indicate that the future will demand shorter novels. I acknowledge this, but I also believe that if your book is dazzling, it will sell no matter what length it is.
In the end, you have to go with your instincts as a writer. What are they telling you to do?