Overlord recommends the following for any beginner!
http://www.skywatcher.com/swtinc/pro...1=1&class2=106
But I just read in Patrick Moore's book that he recommends at least an 8" to make out detail on Mars. I'm not sure, I use an 8, but this one right here may have sharper views due to the longer length. It's also cheaper, like 320 bucks at bintel. And it's good for life!