What is the easiest way to add size choices to an apparel product?

There are a few ways you could approach this:
1)  Kitting - You could set up each size as a separate product.  You could then create a kit product for the apparel item.  Set the kit to use pricing for each individual kit component.  If all of the shirts are the same size, then they could all use the same pricing engine.  This works well if you wish to track inventory for each shirt size.  Please see this article for more information about kitting:  http://support.aleyant.com/kb/a567/ch_-055-products-kitting.aspx
2)  Pricing Engine - You could use a single product and create a new pricing calculator pricing engine.  The pricing calculator would have a quantity parameter for each size.  Note - you can only have 5 quantity parameters, so you would be limited to 5 sizes.  Here is an example of how to set up a pricing calculator with only quantities:  http://support.aleyant.com/kb/a52/how-can-i-have-pricing-engine-where-user-only-selects.aspx  You would just adjust this by creating a quantity parameter for each size and repeat step 2 for each quantity parameter setting the "applies to" to Q2, Q3, etc.  You would not be able to track inventory for each size using this method.