When I think of 'robots', images come to mind of human sized contraptions - something on the order of Tobor, or Robbie the Robot, or even the Jetson's robotic maid. But that's just my background, conditioning, and to some extent bias. I'm a 'big' guy, and I don't have a lot of manual dexterity. I tend to unintentionally break small things, sometimes even when I think I'm being extremely careful with them.
Yet, there's no reason why robots can't be huge, or very tiny for that matter. I don't foresee any hobbist building nanobots in his or her garage, but I wouldn't totally rule it out as impossible. However, there are a lot of people out there building extremely small, fully functional, robots. The 'Pearl Robot' from Korea (see left photo) is definitely a case in point. Just to give you an idea of how small it really is - it's running across the keys of a normal office phone. Designing a microbot that small is a challenge. Making it fully functional is an even bigger challenge. Designing it to run on a smooth curved surface adds a lot to the complexity. Then to have it climb right over obstacles like the raised keys - really amazing, really cool.