No search facility has been set up for that sort of thing yet, you'll simply have to do the hard slog through name books discarding all options but those with 7 letters.
But as a parent I would caution that there is no magic name ticket to psychological soundness...consider 7-letter bearers Charles Manson, Vincent Van Gogh and Timothy McVeigh. Those 3 alone blow all sorts of holes in the 'theory' ;o)
Devon