It is generally the milking routine and the level of automation that determines the speed of milking, rather than the size of parlour.
You may be aware of some large parlours achieving a milking speed in excess of 300 cows per hour, but this will usually involve more than one operator (eg, 2 operators = 150 cows/person/hour) or a minimal milking routine. There are also examples of 50 point rotary parlours with two operators milking 150 cows per hour, equivalent to 75 cows/person/hour, which is no better than a 16:16 herringbone parlour with a single operator.
A typical milking routine would be marshalling cows in to the collecting yard, loading the parlour with cows, fore milk, dry wipe, attach clusters, auto cluster removal, post spray and allow cows to leave the parlour. This milking routine would typically take 58 seconds, with 3600 seconds per hour giving a maximum milking speed of 62 cows per hour. The use of a backing gate could increase the milking speed to 75 cows per hour by reducing cow loading time.
DairyCo's Parlour Wizard allows you to look at different milking routines together with different levels of automation to see the impact on the milking speed.
For more information on design and costings when considering parlour design please see Dairy Wizard.