This file shows a simple particle system that uses a (blend)texture to change the size of the particles.
To try it out:
Press Alt-A to play the animation
Move the Empty around to see the particle grow and shrink with the empty
Right now, The particle system needs to be set to 'Newtonian Physics' for texture to influence the size, and with global gravity turned off.
It would make sense if the size influence could work with the Physics set to 'None'. That should save some computation time and avoid hacks such as setting gravity influence to 0.
Even better if the particle system can update without having the animation being played-back.