There is an additional feature in the product edit page, in Extra Options tab exactly where you can allow your customer to submit special instructions with his order.
Yes, you can create product basket with WWOB and restrict the customer to a specific number of items and force a minimum of items as well. If your customer have to choose exactly 5 items to complete an order, you can use the value (5) in Min Select and Max Select fields in the product select page for the specific product you want to apply this to.
Yes, this can be easily done by enabling product options from the product edit page. Go to WPAli: WooCommerce Order Builder > Extra Options > check Enable Product Options and select the options you want to add, in this case Size options.