Why can't a CT scan see colon cancer well? How large would it need to be to be seen?

Not Designed for it. Not what a ct scan is designed to do. Usually colon cancer will not be clearly evident on ct until it is at stage iii or stage IV level, since it has to anatomically change the colon to be evident on ct. To get that level of change, it needs to be at minimum a locally advanced tumor. Every once in a while it may be evident at the stage ii point, but very low sensitivity.
See below. A ct scan sees solid structures better than hollow ones. Early colon cancer begins on the inside lining of the colon. Unless there is a mass that may be growing outside the colon wall, or a mass several centimeters in size, then it will not be seen on ct scan. Specialized ct colonography can see inside the colon, but needs to have a large enough growth & still cannot tell if it is benign or not.