Take a look at the documentation here: http://fancybox.net/api
You are using inline content, so make sure you set the autoDimensions option to false along with the width and height settings.
UPDATE: I have successfully tested the following solution:
$('a.fbtag').fancybox({
autoDimensions: false,
height: 300,
width: 400
});
This assumes that the class name of the link you open is "fbtag".
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