AdBannerView.size read only ?

Hi everyone.
I’m trying to implement iAD banner in our application and I’m stuck.
The documentation is not mentioning that size is read only but on compilation Unity says that it is.
Is there a way to set the size of the banner on Unity 4.3.1?

Apple doing Apple stuff.
You can´t change the size of the ads in the iOS 6.0.