The dose for Benadryl is 0.5 to 2 mg. per pound of body weight, given every 8 to 12 hours. I think starting with 50 mg. is what I'd do, but I wouldn't hesitate to up it a little if you don't see results. It can take up to a couple of weeks to really tell if an antihistamine is working. Generic diphenhydramine is fine, you don't have to get the name brand. Make sure it's just diphenhydramine and not a combination product.
Be aware that you may have to try other antihistamines. There are lots of them that dogs can take, but unfortunately no particular antihistamine seems to work well for all dogs. Benadryl is usually recommended first, probably because it's inexpensive and something a lot of people already have on hand. It's never worked very well for any of my dogs. We've always had to do trials of several different antihistamines to find one that works. My Shih Tzu is currently getting Zyrtec to help with his ragweed allergy.
