MGs usually range anywhere from about 25,000 to 35,000+
Chapters occur when you're changing scenes or about to spill the beans (hey, that rhymes!). Read your ms. out loud, and you'll get a feel for when a new chapter could start. And yeah, not too long.
For instance, in my book that's out at publishers now, it's about 32,000 words with twenty-two chapters. That could change. Don't worry too much about the chapters at this point.