We make most products to stock and use quick WOs (an addon) in most cases. But we have one line that is always made to order.
For that line, which is essentially a box of varying size, material has to be manually allocated.  I use all type N items in the BOM just so that the operators will know how many and of what size they should cut the blanks from the larger raw material.  They then tell us what raw material and how much they used for the manual allocation.  
 
Ideally to handle the type N parts in a generic way so that I don't need to create item numbers and descriptions for every possible combination I would like to have 3 part numbers; Wide, Narrow, End.  Then as I build the BOM use those Item numbers but change the Description to indicated the size for this particular parent.  Each parent will consist of 2 wides, 2 narrows and 2 ends.  Examples of parent sizes is 19.5x19x12, 20x19x14, 20.5x18x10.  The number of unique type N parts grows quickly.
 
I considered routings but can't reach my objective. I'm not sure this is the only approach to accomplish my goal.
The goal here is to show the operator the size and quantity of blanks to cut for the order on the traveler.  
I was hoping to find a system default to allow description modification during BOM entry but no such luck; then just use the 3 part numbers but adjust the size in the description line right in the BM-A screen.
 
If anyone has a idea for me to pursue it would be appreciated.

I would set up the three part numbers as you are planning to do.  Make them type A (subassembly) parts.  Create a router for each part number with a blank field in the notes section.  Create a Bill of Material and go ahead and list the type of material that will be used (18 guage cold rolled steel for example) and leave the usage quantity 0.  Now create work orders for the three parts and use WO-K-A and WO-K-B to modify the work orders for the current job you have going through your shop.  You can modify the work order router with the blank sizes you need and modify the work order BOM to relieve inventory of the material that will be used for that job.