iPad Pro will feature 2732×2048 resolution screen
Developer Steve Troughton Smith, who previously discovered that the UIKeyboard can scale to a larger size than before, found this detail in the iOS 9 code.
For the sake of comparison, the current iPad Air 2 features a resolution of 2048×1536 at 9.7-inches. That means that the iPad Pro will nearly have the width resolution of two 9.7-inch iPads running next to each other.