Why do you want to giver her Capstar? Does she have obvious fleas? Your best bet, just to be 100% safe, would be to get her to a vet. She may have some underlying issues that aren't obvious and you definitely don't want to harm her. They can give you an accurate weight and can estimate her age for you as well.
If you can't afford a vet, contact your local rescue and see if they have any low cost/free resources.
Rescue lady will talk to me next week. I hope she can take him to the vet. I can not. But in the meantime he is in my apartment. I am trying to find a foster for him. I can not keep him. He's is skinny. But he might be two pounds. I think they need to be two pounds to give them Capstar. I just gave him a bath, but I don't know if I got all the flea's.
Do you have a food scale, or a human scale? You can use the food scale to see how much we weighs, or you can weight yourself and then hold him while on the scale to see how much he weighs.
I've been told Capstar is safe for even very young kittens, so 2 months should be fine. But of course this is very non expert advice, so do with it what you will lol.
I've read the safety sheets on Capstar, and they were giving cats massive overdoses to test it without getting any bad reactions, so it seems to be super safe. I'd use it. If you're worried you could cut it in half. That's enough for a kitten.
Thanks for all the replies. A lady picked up the kitten to foster it, so it's not my problem any more. But I will remember this information for the next time.