Here's an interesting comparison of the terrestrial planets in our Solar System—left to right Mercury, Venus, Earth and Mars to scale. Looking at the numbers, their radii are: Mercury 2439 km, Venus 6051 km, Earth 6371 km, Mars 3389 km.
I grabbed this from the first video lecture of Dr Bruce Betts' Introduction to Astronomy course. Bruce is from California State University and he's also the guy who does the "What's Up?" sections of the Planetary Society's Planetary Radio podcasts (also available via iTunes).
You can watch all his lectures as they become available each week, or if you can't wait, there's a link on that page and you can dive right into last year's lectures...
Back to the planets with a few more numbers: check out the overall density of each planet.