This question seems to resurface every August. Here are the facts.
Mars made its closest approach to Earth in human history on August 27, 2003. An astronomer back then was trying to explain how big it would appear and reported that the apparent size of Mars in a telescope at 75x would be similar to the apparent size of the moon with the naked eye. That statement was oft repeated but usually misquoted leaving out the part about the telescope. The urban legend of a Mars the size of the full moon was born.
Unfortunately, the emails that were flying around didn’t mention the year, so they have been circling the globe ever since and seem to resurface every August.
As Mars orbits the sun in an elliptical orbit, its distance from Earth’s orbit varies; it moves in and out. Its closest point occurs about every two years.
In 2003, Mars reached opposition, its point in the sky directly opposite the sun from us, at almost the same moment that it was at perihelion, its minimum distance from the sun. That arrangement made its angular size a bit larger than its usual close approach. It’s not all that rare an occurrence, in happens about every 15-17 years, but the event in 2003 just happened to be the closest approach Mars has made in the last 60,000 years. It must be stressed that it was only a little closer than in similar arrangements of the past, and appeared only a tiny bit larger – hardly noticeable in the telescope and certainly not noticeable to the naked eye. We'll get a similar look again in July of 2018 but Mars won't exceed the size it attained in 2003 until the year 2287.
Of course, Mars can never appear as large as the moon from the Earth. The moon has an apparent angular diameter of about 30 arc minutes. At its very largest, Mars can get to about 25 arc seconds in size. A factor of more than 70 times smaller. Mars is usually much smaller than that, in a typical opposition it will be 13-20 arc seconds and at other times can be less than 5 arc seconds in its apparent angular size
The best time to view Mars is around opposition which occurs about every 25 or 26 months. The next opposition of Mars is in April of 2014.