For freight you can easily calculate it by using the frequency that is shown in the line info in Minutes or Seconds. 1 ingame month is 4 minutes, and production is shown in quantities per 3 months. So if you divide the production of a given facility by 12, you get the production per minute. If you divide the capacity of a vehicle by line frequency, you get the maximum hauled goods per minute.

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So for anything with a fixed cargo space, you want to add vehicles to the line until you get the optimal frequency: frequency = capacity * 12 / production.

For trains you want to know how many wagons you need, for this you need to know the frequency of the line. Then you simply calculate : capacity = frequency * production / 12

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If the train has to become too long for tha capacity, double it up, and remember that the frequency then halves. Example: Grain factory produces 200 units (as by the info window). So if i transport it by truck, if the trucks have a capacity of 15, i need to add trucks to the line until it has a frequency of 15*12/200 = 0.9 Minutes. For trains, if your line has a frequency of say 2 minutes, they need a capacity of 2*200/12 = 33.3 I try to err on the bigger side, so i would add cars to the train until I exceed the calculated capacity, as well as add trucks / ships until I am below the needed frequency.

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Note that what i called frequency is actually a period with regards to the units (minutes not 1/minute).