According to this
http://www.google.com/webmasters/+1/button/ it is only possible to define a width for annotation inline (which is not balloon). As the extension currently doesn't take variables for either width or type of annotation it is not possible to make this possible.
You could experiment around with putting in your own code like the link suggests. I tried that too but found out that the width behaves peculiar, if the text to be displayed can't be displayed within the width specified or even when the width is much larger nothing shows up (but that could also be due because I am testing on a testsite that obviously has no +1's).
So I guess you need to do some searching yourself.
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