Pick-and-place Humanoid Robot

We are developing a Pick-and-place humanoid robot and encountered this problem: We realized that the robot must be able to decide if it should use only one hand or both hands in picking a certain object based on how large the object is. We came to think of this because it seems awkward to see the robot pick a small object with both hands, while it may not be suitable for the robot to pick a large object with only one hand. May I ask your opinion on what input device do we need to be able to determine the size of the object?

Also, when the robot "decided" to use only one hand, it has to decide which hand it should use. And the obvious answer is the hand nearer to the object should be used. I thought we should need something to determine how far the object is from the left and right hand and compare its distance. May I ask what should we need to get the object's distance from each arm and do we need an extra circuit to compare this values obtained?

THis is one of the problems I was talking about when I meant you would have to figure out how the robot would use the arm on it's own without a person. To figure how big something is also figures out how far it is. What you need is vision or a imaging rangefinder. Like we said before, if you are asking these questions, you don't know enough to tackle such a project. Start with one arm on wheels.

Thanks for your suggestion. But as much as I want to start at simpler things, I don't have a choice (other than drop the subject of course). This humanoid is a requirement in our subject and it was our professor who had chosen this thing for us. He wants us to be really good by giving us really hard tasks.

I hope he's given you the money for everything too.