I'm working on the Jennic JN5148 module, which is a 32-bit microcontroller with embedded 2.4 GHz transceiver, it's about 1'x0.5'x0.1', so with the battery it will surely fit in your specs (I've seen a very tiny case with it). Plus, it consumes about 15mA * 3V transmitting, and your system requires a very short duration, say 5 ms.
2.4 GHz has the advantage that you require a very small antenna (there are also PCB patches) and you have enough bandwith to do very short transmissions, thus saving power.
In general, you can improve drastically the reliability using acknowledges, but this doubles (at least) the number of signals you are transmitting, at the expense of power. For a simple link like this you could use the plain 802.15.4 physical and MAC protocol, that is well suited for point-to-point transmission and simple networks.
Finally, if your application is allows it, it's much more efficient if you can buffer the data over some cycles (let's say 1 byte instead of 1 bit) because the metadata (header and maybe parity) scales much better with a bigger payload.
I've ran some tests on a sensor node equipping this module: I can confirm (very accurate measurements) that the transmission cycle of this module consumes about \$63 \mu C\$, which means \$63 \mu A\$ for 1 second, or 17,5 nAh; if you have a coin cell battery of 1mAh (which is very small), you can do approximately 57 millions of transmissions. And this is for a 64bit message, you can shrink it more sending only one bit.
Ah, it has a range of about 30m in a closed space with walls and many many devices in the same frequency.