I recently purchased some Parallax PIR Motion Sensors.
These are some of the smallest integrated PIR sensor units you can find, plus they are pretty cheap. They actually are a bit larger than I had thought before I purchased them, but still they are really small compared to the units you typically find for security purposes.
I didn't know what to expect with regards to performance, but I tested them and they were capable of detecting motion out to at least 15 feet away. They take 3.3V to 5V supply voltage and have 3.3V output logic.
Their small size makes them useful for applications where you might want to put them at the end of a tube to create motion detectors with a very thin field of view.