In Bootstrap 3 all you need is this
<style>
.modal .modal-dialog { width: 80%; }
</style>
Most of the above answers did not worked for me !!!
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>_x000D_
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.5/js/bootstrap.min.js"></script>_x000D_
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet" />_x000D_
_x000D_
<style>_x000D_
#myModal1 .modal-dialog {_x000D_
width: 80%;_x000D_
}_x000D_
#myModal2 .modal-dialog {_x000D_
width: 50%;_x000D_
}_x000D_
</style>_x000D_
_x000D_
<button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal1">_x000D_
80%_x000D_
</button>_x000D_
_x000D_
<button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal2">_x000D_
50%_x000D_
</button>_x000D_
<center>_x000D_
<!-- Modal -->_x000D_
<div class="modal fade" id="myModal1" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">_x000D_
<div class="modal-dialog" role="document">_x000D_
<div class="modal-content">_x000D_
<div class="modal-header">_x000D_
<button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">×</span>_x000D_
</button>_x000D_
<h4 class="modal-title" id="myModalLabel">Modal title</h4>_x000D_
</div>_x000D_
<div class="modal-body">_x000D_
custom width : 80%_x000D_
</div>_x000D_
<div class="modal-footer">_x000D_
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>_x000D_
<button type="button" class="btn btn-primary">Save changes</button>_x000D_
</div>_x000D_
</div>_x000D_
</div>_x000D_
</div>_x000D_
_x000D_
_x000D_
<!-- Modal -->_x000D_
<div class="modal fade" id="myModal2" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">_x000D_
<div class="modal-dialog" role="document">_x000D_
<div class="modal-content">_x000D_
<div class="modal-header">_x000D_
<button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">×</span>_x000D_
</button>_x000D_
<h4 class="modal-title" id="myModalLabel">Modal title</h4>_x000D_
</div>_x000D_
<div class="modal-body">_x000D_
custom width : 50%_x000D_
</div>_x000D_
<div class="modal-footer">_x000D_
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>_x000D_
<button type="button" class="btn btn-primary">Save changes</button>_x000D_
</div>_x000D_
</div>_x000D_
</div>_x000D_
</div>_x000D_
_x000D_
</center>
_x000D_