The Ivermectin dose is dependent on the weight of the dog. You MUST weigh him to give a dose that is even close to correct. Most vets have a scale in their office and will allow anyone walking in to weigh your dog. A lot of pet superstores and smaller pet shops also have scales, but call first before you go to make sure.
If nothing else, you can always weigh yourself, weigh the dog as you are holding him (if you are strong enough to pick up and hold your dog), and then subtract your weight.
If your dog is 60 pounds, for example, (27 kilos), he needs 300-600 mcg per kilo, so a total of 16,000 mcg. If you are using the 1% ivermectin for cattle he will need about 0.9cc. I would start him at 0.5 cc (milliliters) the first few days, make sure there are no problems, and then move him up to the higher dose. He may need up to twice that much, but try the smaller dose first and see how he responds.