OT: MAYA particle size based on position

[Gerbrand Nel]

Hey guys. I ask here because this is till where the smart friendly 
people hang out.
I need to change the size of particles based on their current position.
I need to create a grid like effect, so as the particles move through 
space, they scale bigger and smaller based on their position in x and y.
In ICE this is easy, not so much in MAYA.
Anyone have any ideas for me?
Thanks