The dimensions of a leaf is determined by the loading it is under. A leaf spring can be lightly loaded as used in the suspension in a small trailer you would tow behind a car. A leaf spring used in a logging truck will be of heavier construction as it has to deal with larger loading. To specify the size of a particular spring it needs to be designed to the conditions it is required to operate under. As a leaf spring is a form of a beam and from what I can remember, have some similarity with beam design formulae. I have all this somewhere in a textbook or two somewhere so I will see if I can find it tomorrow. You may find something on google