I'm Jennifer Macaire, and I'm the webmistress and cover artist for Calderwood books. I also write for other publishers, and I can answer this question.
50k is standard for e-books - at Ellora's Cave and Loose Id, among others, that's considered novel length. E-books are often slightly shorter than paperback books. Ellora's Cave requires a book be at least 50k in order to be made into a paperback. According to their website, 70k and up is a plus sized novel, and below 35k, it's a novella or short story.
For 'real book' publishers, 80k is the required length for a novel, and 60k is considered normal for YA books (from Medallion Press book length requirements.)
According to their guidelines, Calderwood has no preferred novel length.