One-third-sized pre-cracked Charpy bars (MPCC, a/W ~ 0.5) were tested over a range of temperatures. Initiation by unstable fracture or extensive pop-in/arrest events took place in the low and intermediate temperature/toughness regime; at higher temperatures/toughness levels, crack growth was limited to blunting/local damage. The small specimen toughness curve is shifted down in temperature and has steeper transition slopes compared with data for larger compact tension (CT) specimens from the literature. Full size pre-cracked Charpy (PCC) specimens with a/W ratios of nominally 0.1 and 0.5 were tested at −73°C. The toughness for PCCs with a/W = 0.1 was higher by factors of 2 to 4. Confocal microscopy (CM), fracture reconstruction (FR) and finite element method (FEM) simulations were used to evaluate the local conditions at cleavage fracture. For a/W = 0.5 (PCC and MPCC), initiation took place at a single critical stress-critical area criteria independent of size and temperature. However, for the a/W = 0.1 specimens, the critical stress was lower with a correspondingly larger critical area. This is consistent with the reduced stress field present in shallow cracks. Failure by cleavage initiation at a lower critical stress is tentatively attributed to the effects of higher local strains modifying cleavage conditions.