database model tshirt sizes

Hello,

Does anybody know how to build a database model to include sizes for rings, tshirts, etc?

the current database is built like:

table product
=========

productid int8 PK
productname charvar(255)
quantity int4

what i want now is that WHEN (not all products have multiple sizes) there are multiple sizes available. The sizes are stored into the database. I was wondering to include a extra table:

table sizes:
========
productid int8 FK
size varchar(100)

but then i have a quantity problem. Because now not all size quantities can be stored into this table, because it allready exist in my product table.

How do professionals do it? How do they make their model to include sizes if any available?