If you can succesfully implement a graduated colors renderer then you are almost there. In the IClassBreaksRenderer there is a property for Symbol. Just pass a symbol (which contains the size) into this property for each break index.
I am already setting the size as you have described...
When I open the MXD and look at the layer symbology properties, the "Quantities" Value (Box) shows Graduated Colors. I am trying the set this to Graduated Symbols.
When I set it in ArcMap, I get the rendering effect that I am looking for.
The layer symbology properties dialog now opens with the Graduated Symbols item selected.
Only the settings that show when Graduated Colors is selected are being honored (map, toc, legend).
Also, when I alternate clicking between the two options, Graduated Colors shows the symbols that I set thru ArcObjects. Graduated Symbols is randomly picking a color each time i click on it.
Basically, the sizes and symbols are only being applied to the Graduated Colors configuration.
Glad you got it working. That ILegendInfo class can be confusing because it controls the appearance of layers in the TOC and their renderer in the symbology tab. Don't forget to mark this thread as answered.