You should be able to feel them, but they should not be very large. Their size depends on the breed of dog. Obviously a Great Dane's would be much larger than a Chihuahua's, so it's hard to tell you what size they should be. If you're concerned, see a vet asap. My Westie passed away from lymphatic cancer and his lymph nodes were the size of ping pong / golf balls.

I would check with you vet to be sure. They can also be enlarged if there's a infection, or if the dog is fighting off an illness.