This is one of my favorite movies of all time. It's smart, it's funny, and Hanks does a really good job at being a young kid.
Something parents should be on the lookout for is some sexual situations. Of course Josh is going to be interested in sex; he is only thirteen, after all. But this is amplified by the fact that he's working with a beautiful woman that finds him attractive. At one point in the movie, Josh goes to the woman's house, and they start kissing, and the woman takes her shirt off. Nothing graphic is seen, but she's in her bra and obviously okay with it. And then, curious little Josh grabs her breast, and the rest is alluded to in the following scene.
Something else parents need to know is that language is pretty common throughout the film. Josh and Billy both use bad language; Billy even says f**k at one point. But it's not in the context of "bad words are fun to say"; rather, it's kind of a turning point in the growing up of these two young boys. John Heard's character has a pretty colorful vocabulary, too, and it's used pretty commonly. Some sexual humor exists, too, with John Lovitz explaining promiscuity to a naive, newly adult Josh.
All in all, Big is an excellent film that shows that it's okay to unleash your inner kid, even when the going gets tough. Parents can talk about what it means to be a kid, and why you should never give that part of yourself up.