In the future I will be making a movie where I need a bolt of electricity (think force lightning from star wars) to go far into the distance. Now, I have been testing different effects and I think my best bet will be to go with a line emitter so that I have better control over its length and direction, although I could be wrong. The problem is, in order for it to look like it is going into the distance, one end of the line emitter needs to be much bigger than the other, so that it looks like it is in perspective and I don't know how I would do this. Any help would be appreciated.
There's no way to control the size of particles depending on the point they are at. The only real controls that you can use to simulate perspective and depth is the Zoom graph, but that would be over the entire emitter, and not just at one end or the other.
If you have an editor like After Effects, Shake, Combustion, etc (heck, even Photoshop), you can get that effect by either turning it into a 3D layer and rotating it, or Batch Processing with the Free Transform in PS (Yes ouch, but it'd work).