symbology - Using BOTH size and colour to represent 2 pieces of information on single point in QGIS

I want the colour of the point to represent value from data column x (eg. rating from 1-5) and SIMULTANEOUSLY the size of the point to represent value from data column y (eg. Number of ratings 0-600).

I can swear that I have done this before but now I can't find how to do this in QGIS 2.10. It only gives the options to display using colour OR size.