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Using the advanced method of guesstimating, let's assume the radius of each of the silicone spheres to be at least 20 cm. Applying V=(4/3) x pi r^3 we get a volume of 33510 cc.
You'd need a radius of 10 cm to get down to 4000 cc. This corresponds to 20 cm diameter, or rather, the length of a human hand