Purchasing Paper

What would be the best way to enter the following in the "Create Purchase Order" ( used for ordering cartons for printing process )

I have an item whose basis price is based on a standard size ( ie. 25 x 38 ) . When I order the item I use a different size and just adjust the unit price to reflect the change in the size ( ie. I find the price per given area and multiply by my required size ).

The size that I order is different for most of my orders so for the most part cannnot set that item as a regular item in inventory. At one time I may order 26 x 30 and at another time it could be 26 x 29.75. If that is the case would I need to specify that item as a non-inventory item?

Any help would be appreciated.