Mailables: Dual Input for Dual Sizes

[miles-grant-ibigroup]

          Could you get rid of the checkboxes and just have two inputs? One for small format one for large? 

Having trouble understanding why you'd want to select both checkboxes without being able to control the number for each, but it it makes sense in context I trust you

Originally posted by @amy-corson-ibigroup in #925 (review)