You do not need to use our "PayPal Cart" button codes if you simply want to accept PayPal checkout payments. Those buttons also no longer work correctly in many ways (including its Continue Shopping button) due to recent changes at PayPal's end that we have been unable to work around, so we're removing those button codes from our system soon. We recommend using your standard E-junkie Cart buttons, which will show buyers a PayPal checkout button in the cart, and these buttons will also provide full support for all of our features.
Now, the trouble with the Add to Cart button offering a Size menu is due to the way your site is built in ASP.NET, which wraps every entire page within an HTML form; this is how ASP.NET lets you edit your pages, but it also means that your pages cannot have any other HTML forms within the page, because forms cannot be embedded inside other forms. The button code we provide for products using Variations/Variants use an HTML form to submit the buyer's option selections (e.g. Size) to the shopping cart, so this code will not work as-is in your ASP.NET site.
If you want to offer a size selection, you will need to use a separate Add to Cart button for each size. You could do this by simply creating a separate product for each size, but there's another way you can do it with a single product. Basically, this entails adding the menu name and each selection to the end of each Add to Cart button URL, like so:
That would be the button code you can use for Size: Adult Small -- note where the '&on0=Size&os0=Adult+Small' ('on0' means "option name 0" and 'os0' means "option selection 0") have been added to that button's href= URL. To create a button for other sizes, just change the &os0=Adult+Small part accordingly (use a + wherever you need a space).