I've got a composition where particles are emitted from a point. It looks like the reverse of a black hole - a central, light object that projects orbs. I used the Starfield preset and modified it to get what I wanted. Everything is perfect apart from one thing: the particles are randomnly getting bigger and smaller. I've checked through everything and can't see why they don't maintain a consistent size - it almost looks like it's being done by a wiggle expression, although that's not the case. I've probably overlooked something, or else the fault is occurring because I used the preset as the base.
I tried building the particle effect again, this time without using the Starfield preset, however it doesn't look as good, and the particles move too quickly, despite me adjusting Physics Time Factor.
Does anyone have any ideas? If it's easier, I have a basic AE file if anyone wants to take a look, and I'll email it.
There is no preset called "Starfield", probably you meant "Starflight"... in that case take a look at the Particle>Size over Life curve. Make it completely flat (select the flat preset) and they should not vary in size anymore. Of course, if they move in depth, because of perspective the on-screen size will change.
On the "from scratch" method I'd suggest adjusting Emitter>Velocity rather than PTF.