@Esllam_Rashwan this seems like generic feature which should be built-in rather than implemented by custom fields.
So if you are selling item where quantity can be calculated using length/width, then I think option can be enabled on item and then if such an item is selected on quote, invoice, order etc. then Manager can automatically reveal these fields.
I think there is already topic in ideas related to this which relates to volume which is similar concept. But I’ll add this to ideas too.