Edit Contact form field length (Phone Number Shopify Dawn Theme)

[kknepple]

Using the latest Shopify version and Dawn Theme 13.0.1 I would like to edit the "Phone number" field and the length of it on the off the shelf Contact Form that Shopify makes available.  I would like for the "Phone number" field to be the same width as the "Name" field above it.  How does one go about doing that?  Thanks!