Whether you are in the App or on the website simply click on the item you are interested in and this will take you to the product page.
From there, if you are on the website, you will see information on the right hand side of the page that displays product info and sizes available.
Click on the "Check Store Availability" button as shown here.
Then you will be shown a real time display of where the stock is available and what sizes are where, like this:
From the App its just as simple, the in store availability tab is just at the bottom of your screen.
You can then navigate through sizes by swiping across your screen.
If for any reason you cannot check the style you are after on the website please contact our friendly customer services team on live chat or at firstname.lastname@example.org and they will be able to help you with more information.