If you are unable to make standard line items because every order is custom sized, then you would create a single item with an editable description and units of area.  During order entry, you would have to enter the height/width/area in the description and do the math yourself when you create the invoices, calculate the area, and update the associated price/area if it varies based on area.  You could keep a spreadsheet handy to do this during order entry.

If that is unacceptable, you could make some relatively simple modifications to the order entry to have Height, Width columns and do the calculations for you and update the description to "Description, Height, Width, Area" after the line is entered.  This would not require any database modifications.

So my line items should look like:
Item code, Description, Height, Width, Qty, Area, Price, Discount, Total

Where Total = Area x Price (less discount)
and Area = Qty x Height x Width.

Please advise the way foward.

thanks.

Moosa