While the total length of the average Microwave's wave is about 4.7 - 4.9 inches (12.5 CM) you can further pinpoint the phase of the wave as well both by frequency (playing with that .2 inches in the bandwidth) and phase modulation. This could be further tuned if needed by allowing Microwave ovens to operate in the other ISM band of 5.7 GHZ allowing for 2 inch waves (5.3 CM) or even the 61.25 GHZ band (0.19 inches). Though, as you move up in frequency, you see less penetration as the power is lost faster on the surface of the objects.