This report provides information about bomb threat stand-off distances. This covers all types of explosives from pipe bombs with 5 pounds of TNT to a semi-trailer with 60,000 pounds of TNT. From the text: "This table is for general emergency planning only. A given building's vulnerability to explosions depends on its construction and composition. The data in these tables may not accurately reflect these variables. Some risk will remain for any persons closer than the Outdoor Evacuation Distance."
National Counterterrorism Center: http://www.nctc.gov