Theres a guy around here running 20"s on his xD and he lives in a rougher part of the state, said the rides a lot rougher, but he hasnt had any problems yet as far as i know... just windshields falling off from 157dB stereo.
Wheel and Tire Size Calculator
Id try that out to get an idea of what size tire / rim offset youre going to be wanting to run on 19s. those calculators are the best thing ever invented as far as a general guideline goes