I think it's really a matter of personal preference. I own one that's somewhere in the ballpark of 16 or 18 inches, and while it works, it really gets in the way.
The one I have from MercTailor is considerably smaller. Maybe 10 inches? I can check later. I prefer this one greatly. There are also examples of ones that are much smaller, only enough to cover the hand. I think that's a little too small to do interesting things with it.
Keep in mind that it's not really a "shield" and that you'll be using it more to protect your hands. You want it big enough to protect your sword hand when you put your hands together, but not so big that it gets in the way or prevents some of the grappling moves where you shoot the buckler between or around the other guy's arms.
When they're 15+ inches, they're more like small shields. They have their place, but it's not precisely the same thing, and so you'll have to adjust your technique accordingly.