mushroom cloud-sized implosion
And why would you hyphenate cloud with sized?
Because that's the convention for adding a hyphenated modifier to a compound noun. "Mushroom cloud" is the noun, and when you're combining a noun with an adjective like "sized" to qualify that adjective, you use a hyphen. For example, "He shot me with the
Speaking of bowling ball examples using "-sized" in them, I found a perfect example from Science News: Bowling ball-sized Devil Toad probably ate hatchling dinosaurs
(Come to think of it, there was probably a Slashdot story about that...)
But the rest of your comments are spot-on. Mushroom clouds are generally caused by explosions, and yes, they do come in all sizes, rendering the original author's statement completely meaningless.
For the size of an implosion, I reckon most people would consider the size to be the volume or area affected by the implosion -- same as for the "size" of an explosion. Since an implosion tends to pull in surrounding matter...
I'm sure the author of the summary thought the phrase sounded cool until we all started nit-picking him.