lurker's comment: Alpha radiation can be blocked by a sheet of paper. Beta with a piece of plywood. Gamma radiation takes several inches to feet of lead to defeat. So gamma is the only type of radiation which really is a problem..yes there is no radiation other than gamma which ain't a problem.
Also, size wise:On March 24, 2004, Eglin Air Force Base Munitions Directorate official Kenneth Edwards spoke at the NASA Institute for Advanced Concepts. During the speech, Edwards ostensibly emphasized a potential property of positron weaponry, a type of antimatter weaponry: Unlike thermonuclear weaponry, positron weaponry would leave behind "no nuclear residue", such as the nuclear fallout generated by the nuclear fission reactions which power nuclear weapons. According to an article in San Francisco Chronicle, Edwards has granted funding specifically for positron weapons technology development, focusing research on ways to store positrons for long periods of time, a significant technical and scientific difficulty.
Doing some quick computations, that's about 18.8 megatons of TNT per pound of anti-matter. For the record, the largest nuclear weapon ever tested was approximately 57 megatons in size. You wouldn't even need missiles to deliever the stuff, suitcases or small packages would work fine.One millionth of a gram of positrons contain as much energy as 37.8 kilograms (83 pounds) of TNT, according to Edwards' March speech. A simple calculation, then, shows that about 50-millionths of a gram could generate a blast equal to the explosion (roughly 4,000 pounds of TNT, according to the FBI) at the Alfred P. Murrah Federal Building in Oklahoma City in 1995.