bootstrap tutorial - How to change the default width of Bootstrap modal box - bootstrap examples - bootstrap templates - bootstrap 3
Answer: Set width for .modal-dialog element
By default, Bootstrap modal are available in three different sizes — default, small and large. However, if you still want to customize the size of the modal window you can override the CSS width property of the .modal-dialog class to resize the default modal. Similarily, you can override the width property of .modal-sm and .modal-lg class to resize the small and large modal box respectively. Here's an example:
Example
As you can see in the above example the modal classes are placed inside the CSS media query so that modal resizing is only applicable when the viewport has a certain width to accommodate the modal window. When setting the custom width for the modal you have to keep in mind that it should be greater than the viewport width.