I make these points in the thread "Why do you steel my screen"
http://www.emaildiscussions.com/showthread.php?t=65752
There is room for the left hand side bar showing the folder list, and the rest of the screen needs to be devoted to the read or compose panes. There is no room for a right hand side bar with wizzies, gizmos, calendars or anything else.
This gives the width for an optimum 80 character line length when using a font big enough for comfortable reading on a small screen.
I can achieve this in the new classic interface with some browser zoom. Finding a solution in the new interface is not possible because the read and compose feature is locked into a frame that stops two thirds across my small screen.
Gmail has the same problem.
There must be many who will want to use the Fastmail web interface on a 10" tablet or a laptop with and 11" or 12" screen. We need either a more varied range of style sheets or (better) other means to configure screen layout and font size. The vertical use of space is just as critical as the horizontal.
I hope you are reading and listening, Devs.
My ultra-portable is dying and will soon be replaced with the £229 ARM Chromebook with an 11.5" screen. My plan had been stick with old Fastmail, new classic will I think be usable, but it could be better.
PS It is rather confusing to have an interface called "Classic" and a stylesheet called "Classic".