There is a need for more explosion relief data in large L/D vessels. In this paper we report an experimental investigation of the effect of vent size on the overpressure development in a large L/D, end-vented vessel. Three pressure peaks were identified: P-1 due to an initial elongated flame (characteristic of large L/D explosions); P-2 due to turbulent combustion in the middle and end sections of the vessel and P-3 due to the external explosion. P-1 was found to be the dominant overpressure for small vent areas and to increase with decreasing vent size, whereas the flame speed was reduced (as well as P-2 and P-3) due to reduction of unburnt gas flow through and out of the vessel.