I agree with x100x, setting RAW looks like it overides any Size setting and defaults to L (12 Mp).
As an experiment I set M size first, then switched to RAW (which incidentally then blanks out the Size setting function) and compared this with a similar image after setting L size then RAW. Both images were the same file size.
After developing both images to JPG in Silkypix using identical parameters I couldn't detect any differences between them either in terms of resolution, noise or highlight recovery or anything else.
Apart from this I found this article very helpful.