Hi Stuart,
It is not possible to set height or width for the cell. The way that the HeatMap works is that it checks to see how much space it has (actual width and height) and draws the cells so that all cells have equal size and so that no empty space is left. If you know how many rows you have - you can set a Height to the HeatMap or ColorGrid depending on that number.
Greetings,
Petar Marchev
the Telerik team