I have been using the following to increase the width of my text box on a custom form using SPD 2013.
<style>
textarea.ms-long{width: 400px;}
</style>
What is the class / definition for a single line text box for a custom list form?
Single line text fields have "input" tags and not "textarea". So the style should be applied like this:
<style>
input.ms-long{width: 400px;}
</style>
<!--apply the class "ms-long" to the input field-->
<input class="ms-long" id="yourID">