Concept:
A pedal operated thresher is a manually operated threshing machine used for separating grains from crop heads.
Step 1: The cylinder is the main threshing part of the thresher.
Step 2: The size of the cylinder depends on the operating capacity and number of persons operating the machine.
Step 3: When the pedal operated thresher is operated by two persons, a larger cylinder length is used for effective threshing.
Step 4: The standard length of cylinder in such condition is:
\[
700\,mm
\]
Step 5: Therefore, the correct answer is \(700\,mm\).
\[
\boxed{700\,mm}
\]