The WooCommerce Advanced Quantity plugin lets you control how the quantity field for products is working.
Right now (v. 1.5.6) you can change the minimum value of the quantity field, you can set a maximum value, you can change the interval between each step in the quantity field, you can set a standard value for the quantity fields and you can add a price suffix.
You can allow decimal numbers as quantity (ideal for shops that sells food, textile or other products measured by weight or length/height – piece goods). For example, allow your customers to buy a tenth of a product; 0.1, 0.2, 0.3, …, or only let them buy half products: 0.5, 1, 1,5, ….
You can decide an interval between each steps, allowing you to control how many products the customer has to buy at a time (ideal for shops with products packed in packages containing x products each – packed products). For example, make sure that the customer only buyes 5, 10, 15, … pieces of the product.