The 11m is my favorite size. Faster than the 13m, but the unhooking ability is off the charts. I work for Best so I'm biased
With C-kites it's super important to space them closely together when you build out your quiver. Jumping from a 9m to a 13m isn't recommended. Trimming the depower is a huge no-no, you lose out on the performance and steering, which is the reason you're riding a C-kite in the first place. Definitely hit me up if you have any additional questions, more than happy to help!