Lne width set in time rather than mm or cm?

[Clifford Racz]

I am transcribing music for some music books and I am generating EPS 
files using lilypond-book.

I read the manual and looked at the newsgroup, seeing that line widths 
are set using a block that defines the paper width:
\paper{
  line-width=120\mm
}

I wondered if there was a way to set the line width so that the score is 
spaced somewhat evenly, but the with would vary with the length of the 
score, and not set to some arbitrary size (like 120mm)?

In the documentation here:
http://lilypond.org/doc/v2.10/Documentation/user/lilypond/Music-fragment-options

There only lengths can only be specified in [cm | mm | in | pt].

Like to say [1 quarter-notes per cm] or something like that?