Depending on how many people you need to feed, you should easily fit a 20# bird on a typical ~22.5 UDS. That said, it is often said that you should not buy turkeys over 14#s because by the time the breast is cooked properly, the other parts are dry.
2013 Stumps Classic - Delivered 4/15/13
Large BGE - Rescued 2012
2010 WSM 22.5"
2010 Weber genesis gas grill
Rescued 22.5 Weber One-touch gold
BBQ Guru CyberQ
2011 Built UDS - Retired