1 for a chi you need to give childrens benadyl not adult.
2 you really need your vet to tell you exactly the amount b/c of the weight of your dog.
3 the benadryl will make him/her sleepy.
4 they need to tell you if it an allergy or cold. cold might need antibiotics, etc
5 w/ all that said, I can tell you that Baby a 7+ lbs chi get btwn 1/2 to 1 tsp when she has a long car ride. anything else makes her too loopy. Now this is just to calm her down not for allergies, plus she is a bit heavier than your chi, but at least you have an idea when the vet tell you. I use a measuring cup and then use a syringe to give it to her a little at a time. I have to really hold her and I don't want her to choke so I give a lttle let her breathe then a little more.
Has the vet ever said why your chi is getting stuffy? Is it allergies?
My vet will usually let me know stuff like this over the phone if it is minor but they usually tell me that if it last more than X number of days bring her in.