If you can shrink and expand stuff instantly, the thing getting bigger or smaller is the least useful part of that spell. You wouldn’t even need gunpowder to launch stuff, put a shrunk cannonball against a wall, expand it, hope the wall holds and it’s the cannonball that has to accelerate to light speed to not be in the same place as the wall. Or get a giant explosion, that’s more likely.
You’ll notice the gas, but the water comes from the acid being neutralized and won’t really be detectable. You just drank a glass of water too, that’s way more water than a tablespoon of baking soda can produce.