Smaller UAVs might be hard to recognize among birds and other clutter. And you can probably apply some stealth technology to increase your chances of staying undetected.
As far as assasination machines go, an insect-sized UAV with a tiny dose of poison would be hard to beat.
One the size of a large bird could park itself on a glass window, then detonate a shaped charge that could send a jet of superhot plasma through the bulletproof glass and kill a person.
Whats the radar signature of something the size of my abdomen flying ten-twenty meters over the city streets?