Free Text field or overtyping item description

we have variati ons of our products that allow custom sizes to be made, we use the standard product as the basic BOM and edit from there, but when i do an estimate i want to be able to change the description without having to make a new item, for example:

Item might be Square Thing 1200, but the customer might order one that is smaller, so i might want to type Square Thing 1000 and not have it show up on the quote as Square Thing 1200 Square Thing 1000

does that make sense and can anyone assist? we have been using a sstem that allows us to hide the item and replace the description with a freshly typed element.