Measuring the resistance of a person's abdomen will not be a good method to determine their girth because the resistivity varies by more than two orders of magnitude. According to the Wikipedia article on electric shock:
The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 Ohms."
So, the resistance varies by a factor of ~100 while the measurement you are trying to make varies by a factor of ~2. That gives a very terrible signal to noise ratio.
Some other ideas that might be more effective:
- Optical: Use a camera against a scale or use lasers.
- Electrical: Determine the resonance frequency of their abdomen (I believe this will be less noisy, but I'm not 100% sure).
- Pressure waves: Bang their stomach with a quick tap and measure the time it takes the wave to propagate around to the back (this might not work well on muscular people).