Not sure if you're still looking for a pattern, but check out the ones by Connie Crawford for Butterick. Her patterns go up to very large sizes (6x) and they have extended instructions to help with the construction of the garments. Right now they are having a sale online, $3.99 for all patterns:
http://butterick.mccall.com/. I'm a plus size too so I feel your pain about the cheap shirts shrinking. It has happened to me many times as well.