Nowhere, Earth itself doesn't have enough plutonium or uranium in it's crust to produce a 35 Gt gun-type nuclear weapon, but there's always Fusion bombs....
About a 2. Unless someone monumentally stupid ends up running one of the nuclear countries, no one would be dumb enough to go to nuclear war. It would cause mass extinction accross the entire planet, even if the bombs didn't hit every where. The only reason we still have nukes is to make sure no one else uses them. Anyone with the power to use the bomb know that if they do use it, retaliation will be swift and fatal.
Technically, we've already had a nuclear war. The Cold War.
and the only reason WW2 didn't turn into a nuclear war is because the U.S. was the only ones with bombs.
One GRAM of antimatter is equivalent to 3 times the bomb dropped on Hiroshima.
I'd say a mass the size of a small shed could do it.
ounce*
Besides, we can't even produce it in bulky amounts, the electric bill to creating one ounce of anti-matter is 25 trillion times the GDP (Gross Domestic Product) of USA.
Maybe half of a continent, basically something like a megacomet impact.