Smallest Planet: Mercury
There are a couple of different ways to measure how “big” something is. The first is an object’s mass (how much matter it contains) and the second is its volume (how much space it takes up). The smallest planet in regards to both mass and volume is Mercury — at 4,879 km across and 3.3010 x 1023 kg, this tiny world is nearly 20 times less massive than Earth, and its diameter is about 2½ times smaller. In fact, Mercury is closer in size to our Moon than to Earth.
(In case you're wondering, though, Mercury is still significantly larger than the dwarf planet Pluto: Pluto's equatorial diameter is just 2,302 km, about half Mercury's width.)
Largest Planet: Jupiter
The largest planet in our solar system by far is Jupiter, which beats out all the other planets in both mass and volume. Jupiter’s mass is more than 300 times that of Earth, and its diameter, at 140,000 km, is about 11 times Earth’s diameter. (Jupiter's Great Red Spot, even at its current diminished size, spans 15,900, just over a full Earth diameter.) Jupiter is 2½ times more massive than the rest of the planets in the solar system combined. Despite its bulk, though, Jupiter has a fast rotation period of just 10 hours!