How can I change the default width of a Twitter Bootstrap modal box?

Answer 1

If you want something that does not break the relative design try this:

body .modal {
  width: 90%; /* desired relative width */
  left: 5%; /* (100%-width)/2 */
  /* place center */
  margin-left:auto;
  margin-right:auto; 
}

As, @Marco Johannesen says, the "body" before the selector is so that it takes a higher priority than the default.