1) You can use UILexicon, which is technically a dictionary of key-value pairs of strings wrapped in a UILExiconEntry object
Apple give the example of "Iphone" turning into "iPhone" and "Nur" turning into "Nur N. Eytan" (from your address book) etc..
But you can make smarter things.
One smart thing for example would be to create an open source project that takes in a JSON of strings and converts it to UILexicon :) I'd use it!
2) Apple give you 320 pixels width and ±215 pixels height for your
keyboard.
You are prohibited and technically can't draw outside of these
boundaries. What all the companies releasing custom keyboards did was
reduce the size of the keys in 20%-25% in order to add a candidate
view (predictive text bar)
EDIT: you can adjust the height of your keyboard on iOS8.0 - here's a good pointer:
iOS 8 Custom Keyboard: Changing the Height
Good luck!