Guess I should have kept playing before replying to my own post, for others here is the answer on the sizing:

Click on the Series in your chart, select Custom Attributes and there is a MaxPixels property. Set that to whatever number you want, and it will only be that big. I think PointWidth also has something to do with it, default value for it is 0.8.

Sorry for the noise, but I learned something. Hopefully someone else might also.