One block, one torch on top. The simplest redstone logic there is. Footprint 1 wide × 3 deep × 2 tall. Build it block by block in Minecraft.
How to build it
The photo shows the build from the side:
Place a lever at the bottom — that's your input A.
Run redstone dust up one block to reach the side of a solid block.
Powering this block from the side via dust is what will drive the
inverter.
Put a redstone torch on top of that block.
Run dust off the top of the block, up to whatever consumes the
output — here it's a note block so you can hear the signal click on
and off.
Why it works
A redstone torch is on by default and turns off as soon as the block
it's attached to is powered. Flip the lever:
Lever off → block unpowered → torch on → output on.
Lever on → block powered → torch off → output off.
One block, one torch, one inversion.
Gotchas
A torch can't power the block it's sitting on. Its signal goes to the
block above it and to any adjacent dust. That's why the output dust
in the photo is on the top face, not inside the block itself.
Over long distances dust loses one strength per block and dies at 15.
Add a repeater on the output lead if you need to run it far.