The scales I mentioned (i.e. mongo seed, marble, etc.) were based on Bill Nye's video. But the calculation for scales are easily based on one reference point: Distance of Earth to sun.
And thus, using this scale...
149,600,000 kilometers (ave.) = 100 meters = 100,000 millimeters
So doing 149,600,000 / 100,000 we get 1,496...which when used...
149,600,000 / 1,496 = 100,000 millimeters
Now when we substitute the Earth-sun distance with planetary diameters, we easily arrive at the planetary diameters in millimeters:
Mercury = 4,879 km = 3.2 millimeters
Venus = 12,103.6 km = 8 millimeters (actually smaller than a marble, which is about 10-12 mm, so just 2mm bigger than an airsoft BB pellet!)
Earth = 12,756.2 km = 8.5 millimeters (almost as thick as a 9mm bullet)
Mars = 6,794 km = 4.5 millimeters
Jupiter = 142,984 km = 95.5 millimeters (actually larger than tennis ball, which is only about 65 millimeters)