Height and width of the subgrid from xml

How to control the height and width of the subgrid with in a grid? I tried using enableAutoHeight function but doesn’t seem to work.

For example: My main grid is 950 px wide and 95% high want my subgrid to be 800px wide and 400px high how do i acheive this?

done through xml preferred

In order to control subgrid height you can use the approach and attached file from the  following reply: