How big would a tumor have to be to cause a midline shift or mass effect in the brain? is this what rads. look for on cts or the actual tumor?

Look for everything: Depending on the clinical presentation, you would generate some possibilities- you would have to evaluate everything- is there any bleeding, is there any signs of ischemia, is there any mass ( if the contrast is used), if there is a mass- where is the location, how big it is, is there any mass effect, edema, bleed around it etc. Remember head is a close space-any mass can increase pressure inside.

Depends: Midline shift can be caused by a tumor or by the edema (or swelling) around it so there is no specific size that will cause midline shift. A ct scan is to look for mass effect, swelling and bleeding.