We get this question a lot - what is the required photo size for Flexmls?!
The answer is that there really isn't a "required" photo size. You don't want to exceed the maximum, which is no larger than 15 MB and no more than 3000 x 2000 or 2000 x 3000 for the resolution.
There really isn't a minimum, so it all comes down to what you like best.
Flexmls is built to make your photos easy to view, so it will adjust to the majority of uploads you can throw at it, and make your photos shine.
NOTE: if you are syndicating your image to sites like Zillow, Trulia, etc, and your images look fuzzy there, but fine in Flexmls, they may not meet those sites' minimum standards, which vary, but hover around 330 x 220 pixels minimum size.