Historically, the taller candidate has won the Presidency far more often than the shorter one. (See Comparative table of heights of United States presidential candidates for the data.)
The modern exceptions to this rule are GW Bush, Jimmy Carter and Richard Nixon — and they were all bad Presidents.
Barack Obama: 6' 1½” (1.87m)
John McCain: 5' 7” (1.7m)
Hilary Clinton: 5' 6” (1.68m)
It follows, therefore, that the Democrats should nominate Obama, or something terrible will happen.