I've just been up in the loft with a tape measure to find my dish:
dish width is ~53 cm / 21 in
dish height at the join between the 2 reflectors is ~48.5 cm / 19 in
The reason for the 2 halves is that although it was amplitude comparison monopulse in the vertical plane, it was phase comparison in the horizontal plane - the L-R feed horns thus needed to be about 3 ish wavelengths apart.
