I am having a hard time seeing how a bubble chart, where the bubbles are not sized relative to each other, would be of any use.
You should be able to use the minSize and maxSize options, however, to control them the way that you need to:
bubble: {
minSize:2,
maxSize:50
}
{{edit: I don't see them in the docs either, actually. But you can see an example here: http://jsfiddle.net/fXzke/13/ use either number as pixel value, or string with percent of chart height }}