Glad to hear that you like the add-on.

Unfortunately, the size option is limited to what Firefox options are currently in Firefox. At the moment, they just offer the two, but I'm hoping they will eventually add more custom settings.

As for the site-specific settings, that's on my to-do list.

Glad to hear you like the extension!

Currently, it's not possible to add a fully custom width. Firefox only provides regular, thin or none. Hopefully custom widths are something that Firefox will support in the future.