Set a fixed width for a bar

[paolo prete]

Hello,

In the snippet below, is there a way to force bar 2 (e' e' e' e')  to have 

1) a fixed width with a given number 

2) a fixed width = 40% of the entire staff

...?

Thanks

{

  c' c' c' c' | \noBreak

  e' e' e' e' | \noBreak

  f' f' f' f' | \break

  r1

}