I'm sure it has a method for selecting the different sizes available. Anyway can not remember what it is, but have a look first at the PrepareImageCaptureL and the size index in it. Then if it doesn't wotk, have a look at the SDK docs and try out different options.
You should note that the size index differs on different models. In my app it took much bigger picture on 7610 than 6600 with same size index. What you should do is get all size indexes in code and go trough in loop what size index corresponds wanted resolution in specified phone. (Check TCameraInfo)
Well if you intend to run your app just in one model then you can hard code it of course.