In this guide
Linear dimension - This means taking the measurement in a straight line, starting from the edge of the antenna to the opposite edge of the antenna. The measurement should only include the antenna itself and not any attachment needed to fix it to the wall or roof, or connect it up to your equipment.
Projecting feed element - In a dish antenna, the incoming signals are received by the dish which then 'reflects' the signal into a central 'feed horn'. This is usually positioned at a short distance (a few inches) away from the dish and held in place by projecting arm or arms.
Cubic capacity - This means the volume (the amount of in 3 dimensions) occupied by an object using known method of measurement.