This is to confirm you that I have added support for a new option. When you view a message, if you click in Options, you will see an additonal checkbox "Message display with maximum width". If you check that, that will have priority.

For now, the setting applies to when viewing a message. The message will take the maximum spacing area available. This is also auto resize if you resize the browser.

Note that the minimum width depends on the content of the message. Specific options such as message version, stars and toolbar, which are dynamic for each message, thus are not always the same width, have an impact to how much you can diminish the area.

But, at least, it will adjust accordingly to the maximum spacing area available so when changing environment, this will follow.

I will see about additional fine tunings, such as when replying, private messages and so on.