pydriver and halo size is it possible

I was thinking that I could use a pydriver to change the size of my halo’s

so I entered something like

Blender.Get(‘curframe’)/1000 for the particles size

that seem to calculate the value - the problem is that when is comes to rendering the particle size still remains the same even though the frame of animation has increased

is this a bug or is there a better way of doing the same thing

anyone any ideas