Always have handy the un-minified CSS for bootstrap so you can see what styles they have on their components, then create a CSS file AFTER it, if you don't use LESS and over-write their mixins or whatever
This is the default modal css for 768px and up:
@media (min-width: 768px) {
.modal-dialog {
width: 600px;
margin: 30px auto;
}
...
}
They have a class modal-lg for larger widths
@media (min-width: 992px) {
.modal-lg {
width: 900px;
}
}
If you need something twice the 600px size, and something fluid, do something like this in your CSS after the Bootstrap css and assign that class to the modal-dialog.
@media (min-width: 768px) {
.modal-xl {
width: 90%;
max-width:1200px;
}
}
HTML
<div class="modal-dialog modal-xl">
Demo: http://jsbin.com/yefas/1