How "big" objects can WISE and NEOWISE detect? I mean WISE is monitoring near-Earth objects, but cannot see the latest Russian meteor and others. Why can it not detect small objects?
What is the limit of it's infrared detectors?
 A: Well, I don't know anything about the detector you're talking about, but here are some reasons that it's really hard for any observatory (optical, infrared, microwave, radio, x-ray, whatever) to detect things like the asteroid that hit Russia:


*

*They're tiny - All telescopes have a limited resolution, below which we cannot accurately tell what we're looking at. The resolution for optical telescopes are much larger than radio and microwave, but the Russian meteor was about $15$ $m$ in diameter, which would have only been detectable at $\sim 1000$ $km$ away from the Earth's surface. 

*They're fast - The detection and identification of objects in the sky requires us to resolve them (in both time and space) so that we can look at them and figure it out. But this asteroid was moving at about $60,000$ $km/hr$, which is really fast for an object that close to us. It's quite hard to resolve objects moving across the sky that quickly, since the telescopes can't track the object across the sky. 

*They're cold - This is possibly the most important limitation on tracking these objects. When they're cold, they don't emit much radiation, so the only way to detect them is optically (ruling out IR/Microwave/Radio) from reflected light. This means that the only time we can actually see these asteroids are during the night. Even then the above two limitations still make it difficult for extremely small objects. 


This image from Wikipedia

clearly shows how fast things near us move across the sky. The flash across the image is an artificial satellite. 
