Bowker, "the world's leading source for bibliographic information", has just analysed more than 13,000 works of adult fiction published in the US. Some of their findings
"1,550 of [the novels] with an identifiable location were set in England, Scotland, Wales or Ireland. New York topped the list of cities, followed by London, Los Angeles (including Hollywood), Chicago, San Francisco and Washington D.C."
"The major fictional genres of romance, mystery & detective, science fiction and westerns constituted half of all adult fiction published in the U.S. The dominant format for the fiction genre continues to be mass market and non-mass market paperback, with 65% of romance, 61% of science-fiction and 58% of mystery & detective titles published as paperbacks."
"The average science-fiction novel was 329 pages long, followed by romance at 324, mystery & detective at 292, and westerns a relatively skinny 261 pages."
So if you're writing a novel about people falling in love in New York that plays out at a little over 300 pages, congratulations -- you've just upped your chances of publication. As a mass market paperback, of course.