The Navy website also lists "17x18" as the dimensions of "the antenna" here:
http://www.history.navy.mil/photos/weap ... s/cxam.htm
This site also supports the 17x18 measurement:
http://pwencycl.kgbudge.com/C/x/CXAM1_a ... _radar.htm
... and it references Friedman as well as a few other works.
Friedman's is published work considered to be the top of the reference game when it comes to this stuff. The bottom line is that we will probably never truly know the real life dimensions of CXAM without actually being able to measure a real set (I think Tim had found one at a museum).