Measurements can be in any convenient units. Simply type the measurement units into the 'units' box, and use these units in calculating the total view area.
To set the scale, place an object of known area, similar in size to the products, on the table and measure it. and adjust the 'Table Area' value to compensate, e.g. if the measured size was 10% too small, increase the value by 10%. Close the 'Settings' dialog and try again. Repeat until the correct value is obtained. It may be helpful to move the object around and take an average of the readings in various positions and orientations to allow for lens distortion.
The Table Area value represents the total area visible by the camera.