This paper provides comparison of initiation toughness values and J^R curve data for a series of geometrically scaled compact (CT) and single edge notched bend (SENB) geometry specimens taken from a well characterized A533B-1 pressure vessel steel. The CT specimens (W = 2B) were of thickness B = 100, 40, 20, 15, 12.5 & 10 mm, and SENB specimens (W = 2B and W = B) of B = 20, 10 & 5 mm. The data reported are multi-specimen J-Δa values obtained from a total in excess of 400 tests made at ambient temperature (20°C) and 288°C on both plain-sided and sidegrooved specimens. The findings of the work indicate that for the material under study there is little effect of specimen size or geometry on the initiation toughness for a given level of sidegrooving. It also indicates that small size specimens (i.e., W = 10, B = 5mm) can provide J^R curve data equivalent to those obtained for the larger specimens for levels of J up to 600kJm^-2 and crack extension Δa up to approximately 25% of their initial remaining ligament (40% for 20% side grooved specimens). Suggestions are made to relax the current specimen size validity limits. The results presented here encourage the use of SENB specimens possibly as small as 5 × 5 mm for the determination of initiation toughness in these materials