Bubble chart: label for the bubble size?

If you right-click on the chart, you will see an option for Chart Options. Most of these are also available as you walk thru the chart wizard. Personally, I would either add a label as the chart title or as one of the axis desriptions detailing what each point means.

There are some guidelines for using bubble charts here

http://office.microsoft.com/en-us/excel/HA011170761033.aspx?pid=CL100570551033

It seems that most bubble charts are for financial data, so the correlation of the size of the bubble generally relates to the total % that the data point takes up in the series.

Turn on data labels (to show X or Y) then right click on the data label to "Format Data Labels..." Label options then appear that allow you to change what the label contains (i.e. change it to show bubble size)

If you right click on data label there is an option to Format Data Labels. You will need to turn on the data labels to see this.