Apparent bee sting, how much benadryl do I give him?

Size?
Wieght?

1mg for each pound the dog weighs.

They come in 25 mg tablets so you will more than probably have to cut the tablets accordingly.

If you notice the wound getting worse or the dog having any breathing difficulties or any other odd behavior get it to the vet immediately.

I wouldn't give the dog any pill. If it is a honeybee sting, get the stinger out of the area and then treat with a paste of moistened baking soda on the area. If the dog does not respond, take to vet.

I would not give your dog anything without at least calling your vet first. Just like a person a dog can have adverse effects to medications and your vet will know your dogs medical history, whether it is safe for your dog, and what dosage is appropriate if it is safe.