Approximately four meters wide
Based on Johnny's answer, we can see that a Creeper explodes outward, in ideal situations, roughly 3 blocks from the position in which it explodes (give or take about half a block). If we include the original block the Creeper is in, that's a 4-block 'radius' in the four cardinal directions. Cutting that in half would yeild a 2-block radius. Though I have no image to reference for the Plasma Cannon, your rough estimate for the blast radius is 2 Blocks, or roughly all blocks surrounding the point of impact. The force upward will likely be greater than the force downward, since each block of ground has to be penetrated.
Now, esince the user ased for actual measurements...a Minecraft block is 1 meter tall, wide and deep. So if we were to smooth out the Minecraft world, what we'd end up with is a blast radius of approximately 2 meters, plus or minus about half a meter, which means the blast is somewhere between 3 and 5 meters wide - approximately 4 meters wide.