Hello Brandon,
Thank you for interesting in our EShop Shopping Cart extension. Regarding your questions:
1. Yes, EShop supports Authorize.net as one of default payment gateways.
2. Yes, Product Options allows customer to choose the Size with corresponding price. You can check a demo product for the same feature on our demo site here:
joomdonationdemo.com/eshop/cameras/digit...non-20d-digital.html
If you have any more questions, don't hesitate to let me know.
Sincerely, Giang