Not directly from the XML im afraid, but if you're willing to create a custom CSS file for your app, then yes.
In your app, create an application.css file (at /etc/<appname>/appserver/static/application.css)
and put this in it:
.SingleValueHolder {
width:250px;
}
or if you want them to be as wide as possible
.SingleValueHolder {
width:auto;
}
If you want to customize them further, I recommend installing the Firefox extension called Firebug, and using the inspect mode to look at what CSS rules are applying from the Splunk CSS. This will help you know what styles to create in your CSS to counteract or tweak the default appearance.
