What you ask for is quite possible. You can have options for the size, and the quantity. You can even define many other things. However, these are not as easy for the non experienced.
A PayPal button is nothing more or less than a form. The code you retrieve from PayPAl when you create a Add to Cart or a Buy Now button, is simply a form code. Seen this way, if you have some experience with forms, and Javascript, you can define whatever parameter you like, like quantity, price, options etc.
Have a look at
http://www.discountwoodblinds.co.uk/dbts_cart/order.php
there , the order form doesn't submit directly to PAyPal, but to the cart instead, however the concept is the same