If you want a good estimate, ...
Published by Bob Foerster, Executive Director at Nevada Rural Water Association
If you want a good estimate, slowly turn a gate valve and count the number of turns between fully open and fully closed. Pipe diameter in inches (sorry about that) is roughly (turns - 3) /3
Or, ask someone who operates the pipeline. Valves should be turned twice annually or they will not work when you need them.
Be aware of water hammer. Return valve to normal position.