For my web site i need change unit to size

For my web site i need change for same product the impostation from unit to size 1 Product change unit to size on (250gr 500gr 1kg 1.5kg 2.5kg) 2 Category change the price on kg (not for all product) 3 When the user change size have to change automatic the price (this for all product) 4 Gestion stock ( the product have to decrement ex 1kg=4 product have to decrement 4 time from stock) [login to view URL] this forum talk about it whit plugin.