How Cart is managed in Sales Order Entry form

I want to know that how Cart items are managed In Sales Order Entry form .

I want to add Size field ( length, width,height) and Sales Order Items cart  look like this

Item Code    Item Description    Quantity    Unit    Length    Width    Height    Size      Price    before Tax    Discount %    Total   

22                        abc                          1            ea     2              3              1        (2*3*1)  1                                       (size * price)