iLevel has great free software for calculating and sizing microlam and parallam lumber.
The reason that the lumber is not usually made to dimensional sizes is twofold: first, laminated lumber uses special fasteners and hardware connectors that are engineered specifically for the loads the member is intended to support. Use of dimensional lumber connectors would not be adequate (or too much) for those loads.
Second, engineered wood is just that: engineered. It's designed for specific conditions and loads (shear, torsion, etc.), and as such, is made only as large as it needs to be to adequately handle those loads. There is no waste. Dimensional lumber is a little larger to compensate for the checks, knots and other natural "imperfections" that affect it's loading abilities.
I guess there's a third, and that's the dummy factor. I don't know if this is intentional (probably not, and covered by the first condition I mentioned), and that's simply that if you try to use a dimensional lumber connector on engineered wood, it just won't fit (in most cases, not all), hence, most people would go get the correct connector (one would hope...we've all seen some amazing things done in residential construction... :-D).
Hope this info helps.
