I usually use Rentometer.com or Zilpy.com to find rents and then cross reference them with a CMA from the MLS. You could also look on Craigslist and see what other people are renting out similar homes for. If you need a CMA let me know. Contact me at 760-583-2735 or go to http://MikeKnowsSanDiego.com
It's not perfect, and any numbers should be verified by a CMA from a Realtor (you can do CMAs on rentals, too). But I've found it actually gives a pretty good answer most of the time.
Hope that helps.
As a general rule, the bigger the house, the lower rent per square foot. In other words, a 1400 sq ft will rent for more per square foot than a 3200 sq ft house.
Best of luck to you!