I have several firends with, and have been acquanted with many large breed dogs. In my expirience, you are more likely to be bitten by a small breed, and attacked by a medium breed. Every large breed, and i mean every, that i have been around tiptoes when small children are brought into the room. I know personally 3 great dane, or great dane mixes and they are the most gentile dogs i have ever been around. A local here used to breed english mastiffs, and the same way, gentile, if they were to harm a child it would only be from knocing them down due to their size and clumsyness.
A guy should not be afraid of a large breed just because of their size. Given all of the various dogs i have owned or had a part of raising, i would take a large breed over any other without pause or second thought.