Sounds like what you are looking for is a way to switch to a different template with a different size depending on what product attribute for size is selected, is that right?
We typically do recommend creating different products for each size, much like our site MakeCanvasPrints where you can see every size is a different product. This keeps the 1-to-1 relationship of a single product with a single MyStyle Template ID, and makes navigation work naturally thru WooCommerce.
That said, you have some options to custom develop a different flow or way of switching the mystyle ID.
You’re right that this would be a custom development or new feature that could probably be a bit of custom javavscript that lives on your product page. You would not necessarily need MyStyle for this if you have your own theme developer.
However another way to do it would be to create a custom product page all together instead of using the WooCommerce one. That way, you can easily code the logic to switch product IDs depending on choices. This is of course a custom development that goes outside of both MyStyle’s plugin and WooCommerce. An example of a custom product page that is custom coded by us would be here: http://www.makecanvasprints.com/canvas-prints/create/
Although we run wordpress and opencart on that site, that page is neither a wordpress nor an opencart page, it is custom coded.
These are two ways to create a custom workflow outside of our base plugin’s default functionality.
If you’d like a quote for us to help with this project, please contact Miguel directly or use the form here to request a quote: