The RTA-4A WX radar fitted to the B737-300's that we fly have a 30 inch flat plate antenna with a beam width of 3.4 degrees.
This is useful in determining the beam width in feet at any given distance.
The formula for calculating this is :
number of degrees of antenna beam x distance (nm) x 100 = beam width dimension (feet)
ie 3.4 degrees x 50nm x 100 = 17,000 feet.