Rich,
I have found that the Kuhl button down shirts are my favorites for CCW and all around wear. My size is somewhere between a L and XL, but with most shirts I lean toward the XL for carry. My Kuhl XL's are not as sloppy looking as other XL shirts, and fit nicely while concealing a handgun. I am sold! I buy most of mine from Zappo's because I hate malls, and wish they were a tad cheaper.
http://www.kuhl.com