[The Might]

Well, I don't want to speculate, but I see them as...well...large men
Perhaps the Giants in earlier writings might have had an ogreish look, due to the fact they were counted among the Uvanimor alongside ogres and trolls.
However, just as Hobbits are a smaller version of Men, Giants are a bigger one, or at least so I see it. As hobbits have their curly hair and hairy feet, giants might have some differences of their own, still I doubt there would be a striking difference. But, as I said it all is speculation.

As for Helm's Deep, I think you misunderstood Tolkien.
"Carved by the hands of giants" or how Helm's Deep (just like Minas Tirith) is described doesn't actually mean that giants helped the Numenoreans, it's just a metaphor to make you picture the size and strength of the fortress.