How to make giant explosion with one command

Creepers have an ExplosionRadius tag to determine the size of the explosion. Because it's a Byte tag, it has a maximum of 127:

/summon Creeper ~ ~1 ~ {ExplosionRadius:127}

Ghasts have an ExplosionPower Integer tag, meaning the maximum value is 2,147,483,647 (which will most certainly crash your game when attempting to use it):

/summon Ghast ~ ~1 ~ {ExplosionPower:2147483647}

The Fireball entity also has an ExplosionPower Integer tag:

/summon Fireball ~ ~1 ~ {ExplosionPower:2147483647}

MinecartTNT, PrimedTnt, and EnderCrystal entities do not have any tags to determine the radius of their explosion.

You can find NBT data for entities here.

And general info on the NBT format (including minimum/maximum values) here.