The readme-files to both version 1.0 and 1.1 of MTW states:

Training Large Units

When you increase the basic size of units they will take longer to train, so that a unit of 200 peasants may take two years to train rather than the default one year.

But how do I do it? I have read through the manual and tried and tried and tried. But I just cannot figure it out.

Any help is appreciated.