Series.BubbleSizes (PowerPoint)

Returns or sets a string that refers to the worksheet cells that contain the x-value, y-value, and size data for the bubble chart.

When you return the cell reference, it will return a string that describes the cells in A1-style notation. To set the size data for the bubble chart, you must use R1C1-style notation.

ActivePresentation.Slides(1).Shapes(1).Chart.FullSeriesCollection(1).BubbleSizes = 10