I found this Chocolate Toxicity Meter on petMD, which enables you to enter your dog's weight, the type of chocolate (milk, dark, etc.) and the amount ingested in ounces. It returns the risk to your dog, along with potential symptoms you might encounter. What can be life-threatening to a little guy might not be a big deal for a large dog.
If in doubt of course, call your vet. Adobe Animal Hospital on El Camino by San Antonio is open 24 hours, as is South Peninsula Emergency Vet, on Middlefield by the Winter Club rink.
My Halloween costume: Palo Alto mom in school drop-off attire, with dorky bike as an accessory. Happy Halloween!