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

Additional class for width forms

Supercharge your Bootstrap modification by incorporating class specificity. Embed a custom class or escalate specificity by using a parent selector like body:

body .modal-dialog {
    width: 70%; /* Effortlessly overruling default */
}
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This strategy guarantees that your custom styles outperform the defaults and maintain the centered position. Here comes the fine-tuned layout!

Responsive for all devices

When it comes to Bootstrap, responsiveness is a must. Reliance on percentage widths or media queries, rather than fixed measurements, guarantees an unmatched user experience across multiple devices:

@media (min-width: 768px) {
  .custom-modal .modal-dialog {
    width: 50%; /* Smaller modals for larger screens, neat huh? */
  }
}
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Aim to cross-verify responsiveness across multiple devices and screen sizes to eliminate any unforeseen layout disruptions.

CSS over JavaScript

Skip the JavaScript or jQuery for modifying modal widths. Instead, CSS is king! It brings about a clean, easy-to-maintain and mistake-free programming.

Centralize your attention

To keep your modals center-aligned, Bootstrap employs horizontal margins. When changing width, it's crucial to manually adjust margin-left to ensure the modal remains centered:

.custom-modal .modal-dialog {
  width: 700px; /* Unflinching width */
  /* Quirk Alert: Don't forget about the pesky scrollbar */
  margin-left: calc(50% - 350px - scrollbarWidth);
}
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For modals with a definite width, leverage calc() for accurate margin values by including the scrollbar width which could cause a visual offset.

Update-safe customizations

For effortless Bootstrap updates, keep your modal customizations secluded in a custom stylesheet. It also streamlines maintainability and circumvents conflicts with future Bootstrap updates.

In-built classes: the atomic route

Bootstrap features inbuilt classes like .modal-lg, .modal-md, and .modal-sm that are ideal for easy sizing modifications:

<div class="modal-dialog modal-lg">
  <!-- Modal content -->
</div>
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Use these classes for common size alterations before diving into custom CSS.

Identifying CSS conflicts

Be vigilant for any CSS conflicts affecting your modal size and alignment. Using browser developer tools can help uncover hidden conflicts that may sabotage your picture-perfect designs.

Feedback: the reality check

After donning your modals with tailor-fitted styles, procure user feedback to identify potential user experience enhancements. This can provide valuable insights for refining modal behaviors.