The Large Trader's Backpack is a heavy weight backpack that is able to stack Trade Goods by at least two items per spot. It multiplies the maximum stack for any stackable item by 3 leading to stack sizes of 6.
Can be found equipped (not sold) by traders at the Holy Armour Shop, Clothing Shop - Holy Nation, Holy Headgear Shop, Holy Boot Shop, and Boot Shops.
They cannot be bought, so to obtain one they must be taken from a Shopkeepers. As it looks similar to the other Traders Backpack in game an easy way to tell if a Shopkeeper is wearing a Traders Backpack Large and not a Shopkeepers good bag is to check their movement speed. If they are not slowed down a ton by encumbrance, then they have a Traders Backpack Large. ru:Вьюк торговца