Bootstrap - changing the width of a form

I have the following code in an ember application for a .hbs template file (basically it's just a chat box for a user to enter text, with "Back" and "Send" buttons either side):

<div class="row">
<div class="chat-form text-left">
<button class="btn btn-default" >Back</button>
<form class="form-inline" >
<div class="form-group">
<label class="sr-only">Chat Message</label>

</div>
<button type="submit" class="btn btn-default">Send</button>
</form>
</div>
</div>

Currently the text box is too narrow and I want to make it a few times wider, I'm aware of columns and spans in bootstrap but as a complete beginner I'm not sure how to do it properly in this case.