**The Size of a Theodolite**

The size of a theodolite is specified by the diameter of the lower plate. The lower plate, also known as the horizontal or circular plate, is a circular disk that is an integral part of the theodolite. It is usually made of metal and is mounted on a sturdy tripod.

**Explanation**

The size of a theodolite is not determined by the length of the telescope, the diameter of the vertical circle, or the diameter of the upper plate. These parameters are important for other aspects of the theodolite's functionality, but they do not define its size.

The length of the telescope is an important specification of a theodolite as it determines the instrument's ability to measure angles accurately. The longer the telescope, the more precise the measurements can be. However, the length of the telescope does not determine the size of the theodolite as a whole.

Similarly, the diameter of the vertical circle is a significant specification that affects the theodolite's accuracy. The vertical circle is responsible for measuring vertical angles, and a larger diameter allows for more precise readings. However, the diameter of the vertical circle does not determine the overall size of the theodolite.

The diameter of the upper plate, also known as the upper motion plate, is the circular platform that holds the telescope and allows for its rotation. It is an essential component of the theodolite, but it does not determine the size of the instrument.

On the other hand, the diameter of the lower plate is the specification that determines the size of the theodolite. The lower plate is the base of the instrument, and its diameter reflects the overall dimensions of the theodolite. A larger lower plate diameter indicates a larger theodolite, while a smaller diameter indicates a smaller instrument.

In conclusion, the size of a theodolite is specified by the diameter of the lower plate. While other parameters such as the length of the telescope, the diameter of the vertical circle, and the diameter of the upper plate are important for different aspects of the theodolite's functionality, they do not determine the overall size of the instrument.

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