On paper, Brett Ratner sounds like such an improbable choice to direct a large-scale ancient Greek epic that, going into his “Hercules,” one could only hope for a less aggressively preposterous affair than Renny Harlin’s bargain-basement “The Legend of Hercules” from earlier this year. The happy surprise is that Ratner’s “Hercules” is more than a mere improvement on its predecessor. It’s a grandly staged, solidly entertaining, old-fashioned adventure movie that does something no other Hercules movie has quite done before: It cuts the mythical son of Zeus down to human size (or as human as you can get while still being played by Dwayne Johnson). The result is a far classier pic than Paramount’s frenetic trailer — and decision to hide the film from reviewers until the 11th hour — foretold, albeit one that will struggle to find its sea legs at a crowded and underperforming summer box office. Overseas prospects look sunnier.