Joomla 3.x use Bootstrap 2.3.2 in the admin area.
So, add a class option to your XML
for your field, e.g. here's the definition of a textarea
note the span6
in the class
attribute:
<field
name="file_exclusions"
label="COM_EASYSTAGING_LOCALSITE_FILE_EXCLUSIONS"
description="COM_EASYSTAGING_LOCALSITE_FILE_EXCLUSIONS_DESC"
type="textarea"
class="inputbox span6"
default=""
rows="10"
cols="50"
/>