Manipilating particle size with respect to movement of my gameobject ?

Noob here. From Space shooter tutorial. Iam trying to increase the paticle size which in my case is a flame behind the spaceship to increase in size when the craft is moving forward(along +z) and decrease in size along (along -z)moving backwards. How do i go about doing this. I tried and played with the size property,but it didn't help.

thanks