It IS possible to change the size of the keyboard in the current iOS 8

Taken verbatim from the documentation: "In iOS 8.0, you can adjust a custom keyboard’s height any time after its primary view initially draws on screen."

To resize your custom keyboard, add a simple layout constraint.

CGFloat _expandedHeight = 500;
NSLayoutConstraint *_heightConstraint = 
    [NSLayoutConstraint constraintWithItem: self.view 
                                 attribute: NSLayoutAttributeHeight 
                                 relatedBy: NSLayoutRelationEqual 
                                    toItem: nil 
                                 attribute: NSLayoutAttributeNotAnAttribute 
                                multiplier: 0.0 
                                  constant: _expandedHeight];
[self.view addConstraint: _heightConstraint];

For more information look at Apple's prerelease documentation here!