Modal width (increase)

For responsive answer.

@media (min-width: 992px) {
  .modal-dialog {
    max-width: 80%;
  }
}

Bootstrap 3

You can create or just override default bootstrap modal-lgby doing below:

.modal-lg {
    max-width: 80%;
}

If not working just add !important so it would look like below

.modal-lg {
    max-width: 80% !important;
}

Now call the modal-lg.

<div class="modal-dialog modal-lg" role="document">
 <!-- some modal content --->
</div

For Bootstrap 4 refer to @kitsu.eb answer. Also note that using bootstrap 4 utilities might break the responsiveness.

Bootstrap 4 includes sizing utilities. You can change the size to 25/50/75/100% of the page width (I wish there were even more increments).

To use these we will replace the modal-lg class. Both the default width and modal-lg use css max-width to control the modal's width, so first add the mw-100 class to effectively disable max-width. Then just add the width class you want, e.g. w-75.

Note that you should place the mw-100 and w-75 classes in the div with the modal-dialog class, not the modal div e.g.,

<div class='modal-dialog mw-100 w-75'>
    ...
</div>

Also, we can even use the well known container class (instead of specifying the size), like:

<div class="modal-dialog mw-100">
    <div class="modal-content container">
        <div class="modal-body">
            ...
        </div>
    </div>
</div>