Ability to choose from multiple label sizes for Fedex

[ggiraudon]

**Is your feature request related to a problem? **
For certain carriers (like Fedex), there are multiple label types and sizes that can be used.
In my case, my printer is set up for PDF_4x6_75 labels (with doc tabs) but the default settings in the dashboard map to PDF_4x6.
This means that I am unable to purchase labels from the dashboard if I want them in the PDF_4x6_75 format.

Describe the solution you'd like
It would be nice to have a way to select other label sizes in the carrier settings.

Describe alternatives you've considered
At this stage, the only option I have is to modify the label mapping in the code directly.

Keep up the good work !
Guillaume