How can you find the image sensor size of the camera?

I want to find how many mm does 1 pixel contain. sensor_size/image_width will give my my mm conversion for 1 pixel. How can I find my image sensor size?

[Kevin Claytor]

If you're asking what the physical size of the sensor is - I would look on the camera spec sheet.

If you're asking "in an image I take, how wide is any given object in the image?" Then the answer depends on how far the object is from the camera. As a simple suggestion, how about using a ruler to calibrate.