Like totaly huge
There is a very interesting article on The Space Review (as usual) on the studies done during the Apollo program on an possible on-pad explosion of the Saturn V booster. The combined explosion of all oboard fuel and oxidizer will have come close to be 1/26 of Hiroshima atomic explosion ... quiet the pad explosion! The abort system, intended to lift the Command Module away from the explosition (and fireball) and designed according to these studies, will have given the crew a very rought ride (8-10 Gs), and an even roughter possible ground landing (see Schirra's concerns in From the Earth to the Moon's episode on Apollo 7) at a speed of 8.53m/s (30.7km/h):
By comparaison, the Soyuz re-entry capsule which was designed for ground landing, have a touchdown speed of about 1.5m/s (5.4km/h) thanks to the use of solid propellant thrusters right before touchdown.
To stay on the same Apollo related subject, make sure to check out these special versions of Wikipedia article on the 1st stage of the Saturn V booster :-)