How big of a mass in the rectum can a pelvic cat scan detect?

Few milimeter or mor. Ct scan can pick rectal mass from few milimeters to a centimeters. Preferably method would be rectal exam with finger/sigmoid/colonoscope.
Depends. It all depends on how the mass involives the rectal wall. If it is srictly intra-luminal arising from the mucosa it may be not be seen well by a ct scan. Endoscopy is better for such lessions.