Assuming you're only wanting change TEC, open these two in particle forge:
Weapon_TechCapitalPlanetBombing_Hit.particle
Weapon_TechFrigatePlanetBombing_Hit.particle
Reduce the size of the particles coming from each emitter accordingly. So for instance: If I have "OrangeGrow" emitter selected and then scroll down to the Particle Section in the table, you'll see a "Size - Width" and "Size - Height" variable. Reduce these to what you want them to be. It would probably be a good idea to change the rest of the values so they're to scale with the main explosion too.
Hope that helps.
