That would be possible but text-only emails are wrapped at 70 characters so if we wrap the preview pane at 60 characters (to prevent the scrollbar), then the line breaks from the email would be at 10 characters into the second line, eg.
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt
ut labore et d
olore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi
ut aliquip ex e
a commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum
dolore eu fugia
t nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia
deserunt mollit
anim id est laborum.
etc.
What I can try to do is increase the percentage size of the preview pane to an absolute minimum of 70 characters, but that will just decrease the amount you have to scroll on smaller screens.
The obvious solution would be to switch to the horizontal preview pane in the next update, which would allow for a wider reading area on smaller screens.