A common misconception when grading a quarterback prospect is "bigger is better." But size isn't everything, and the position has become less about height, weight and speed and more about feel, vision and mental toughness.
A good QB prospect must have at least adequate arm strength and show good zip on the ball. The deep comeback route is the best measurement of arm strength if the quarterback can deliver the comeback throw on a "rope" and without a lot of arc, then his arm strength is at least adequate.
Also, the transition continues from the big, immobile drop-back passer to the player who can buy extra time with his athletic ability. He doesn't have to be Michael Vick, but guys like former Utah QB Alex Smith are what programs would love to have. Today's quarterback must also have leadership qualities and above-average character. The ability to play under pressure is scrutinized, as any team will often mirror the personality of its quarterback. This position requires a tough person, mentally and physically.
That being said, the grading system isn't perfect. Because intangibles are such a factor, many quarterbacks can be mis-evaluated coming out of high school. Plus, these kids are young. Making projections on 17-year-old players two to four years in the future is not an exact science. "System" quarterbacks with great high school numbers also tend to get overrated, just as quarterbacks with little to no statistics get passed by because of the often-used Wing-T or option-oriented offense at the high school level.