You don't necessarily needs to ask permission to edit a model, especially when it comes to something simple like a scaling value. I'll explain it at the end of the post, to avoid boring discussion.
Bob27 is partially correct. To edit a model, you do what he says. (However I'd use Oinerwinkles Vertex Modifier instead, its way easier to use) To edit a particle emitter (Which is what 85% of special effects are) you would want Magos on the other hand. You'd go to Windows>Node Manager, and double click the blue arrow symbol. There may be multiple depending on the complexity of the animation. There should be 3 places in the middle with scale and alpha. Change the scale. If you want it say, 5 times bigger, take the number shown times 5 and put that value instead.
Now that's not all. You may have to adjust length and width as well. All depends on the special effect. Maybe learn a bit about particle emitters, it's pretty simple. THere's a tutorial on it, that's short and simple, in magos help files.
l0git3c has a simple answer, but that would only work in very, very few situations as you may have to deal with play animations to get that to work in your favor.
Alright, now this is the discussion. Do you have to ask for permission when editing a model? No, rarely should you.
First off, if I submitted a model, I wouldn't want every one and their mother asking me: Can I change the size of the model/particle so I can use it in my map? That'd bug the heck out of me and discourage me from submitting models.
If you're gonna use it in your map only, then go ahead and modify it and remember to give credit. Don't worry about permission. You'll keep from bugging the modelers hard at work churning out nice stuff for us.
If you're going to submit a change of a model to a website, or otherwise redistribute someone elses modified work to public domains, you very well should ask for permission for the sake of morality and social ethics. It can stifle the amount of models a person submits when their work is stolen, which hurts everyone, even the person who stole the model. Another reason why giving credit is important. People appreciate it, and it tells them that their work is not being stolen.
But don't bug the modelers for minor edits that'll only appear in your map. I wouldn't want to have to deal with that as a modeler.
K done.