SuicideJunkie wrote:
The size of the warhead is a tactical decision. Anti-tank and fortress demolition were the choices. Planet killer was an option, albeit a bad one.
Planet-killer, not so much.
I'm going to assume you mean "Has a significant impact on the climate of a planet", not "Can destroy a planet." If it's the latter, then the math is easier to do, but the amount of anti-matter needed is large enough to make it's own moon.
So, the former. If we take the Chicxulub meteor (The one that likely wiped out the dinosaurs) to be the amount of explosive needed (it wiped out 95% of all life, so that's probably enough to count as "plant-killer")
Well, that explosion was
1.52 x 10^24 Joules (Use the numbers given to calculate the impact).
1.52x10^24 = (m)3x10^16 [3x10^16 is c^2]
m = 50 700 000kg of antimatter.
A wee bit much.
Let's assume that the heaviest his epaulets are likely to be is 1kg, that gives us a 3x10^16j explosion. Now, that's lots of energy, but not quite on the order of 'planet killer'. (For reference, that's a 7 megaton explosion, the largest nuke ever detonated was 50 megatons)
[/end pedantic rant (complete with math!)]
Edit: The math to blow up a planet is easier because the amount of energy needed to blow it apart is known, the amount needed to destroy the environment isn't (Or, if it is, I've been unable to find it despite extensive searching.)