Decision@Gerrie works, but it applies to all the inputs on the page. If you have other inputs on the page that you do not want to set this way, you can do something like this:
<div id="model-editor">
<%: Html.EditorForModel() %>
</div>
CSS :
#model-editor input { width:220px; }
CSS , . HTML id, :
input[id='YourInputId'] { width:400px; }