Earth and Mars size comparison:

Mars is only about one-half of the size of Earth and yet the red planet has several volcanoes that surpass the scale of the largest terrestrial volcanoes. The four brown spots visible on the northern (upper) portion of the planet are four enormous volcanoes on the Tharsis region of Mars. Olympus Mons is the northernmost volcano and the three aligned volcanoes are the Tharsis Montes. This view emphasizes the fact that the volcanoes of Mars are significant not only as localized centers of eruptive activity but also as prominent landforms on a planetary scale. (Viking Orbiter Views of Mars, NASA SP-441, p. 14, U.S. Govt. Printing Office, Washington, DC.)

More about Mars:

• The diameter of Mars is 4,217 miles, half the size of Earth but twice the size of the moon
• Mars is about 10 light minutes from Earth, so it takes 10 minutes radio signals to travel one way
• Mars is 1.5 Astronomical Units (1 AU=93,000,000 miles) from the sun
• A Martian year is 687 Earth days, or 670 Martian days
• A Martian day is 24 hours, 37 minutes
• Mars gravity is about 3/8 that of Earth
• The poles of Mars are tilted 25 degrees to the solar plane, giving it seasons
• Olympus Mons is 52,000 feet high and 370 miles wide, about the size of Arizona
• Valles Marineris is 2,500 miles long, and on the average 6 miles deep, and 12 miles wide.
• Mars has two tiny moons, Phobos and Deimos. Phobos (fear) and Deimos (terror) were the sons of the greek god of war Ares (Mars in the Roman mythology)

Here's another great size comparison. Phobos and Deimos with asteroid Gaspra.

This montage shows asteroid 951 Gaspra (top) compared with Deimos (lower left) and Phobos (lower right), the moons of Mars. The three bodies are shown at the same scale and nearly the same lighting conditions. Gaspra is about 17 kilometers (10 miles) long.

All three bodies have irregular shapes, due to past catastrophic conditions. However their surfaces appear remarkably different, possibly because of differences in composition but most likely because of very different impact histories.