Pack Size is the number of units per “case”. This is typically how you purchase your product from your Supplier.
E.g., 1 case of beer has 24 bottles. So the pack size would contain 24 bottles when 1 case is purchased.
Item Size is the measure in weight, volume, or "each" of a product.
E.g., 1 bottle of beer is 12 ounces. So the item size would be 12oz or if it’s item size is in eaches, 1 bottle.
Although this is one way of interpreting pack size vs item size, a lot of freedom is given to the Suppliers when it comes to how one determines pack and item sizes. If one does not want to offer an item in small quantities, then they can list the item size to be the same as pack size and vice-versa; if one wants to offer the pack size only, the item and pack size may be listed the same.