It's about the size of a grain of rice. It is implanted between the shoulders usually, the same way a vaccine is given, through a syringe and needle. Brody had his done when he was asleep for his neuter. I've heard some dogs will yelp and some will act like nothing happened. Just depends on the dog. I could feel Brody's for several months after it was implanted under his skin. It had a bit of swelling and the microchip actually encases itself in connective tissue so that it won't migrate. (They have changed them and they have little holes in them so that the tissue "grabs" onto them now instead of just being loose under the skin). It's been about 8 months since Brody had his implanted and I can't feel it anymore.
"Having heard all of this, you may choose to look the other way.....
but you can never say again that you did not know." William Wilberforce 1759-1833