The principal purpose of this investigation was to study specimen size effects on elastic-plastic type R-curve using HY130 steel plate. Compact specimens ranging in size from 12T to 1T in planar dimensions and standardized on 1-inch thickness were used. Using an elastic-plastic concept developed in a companion program, it was shown that the R-curve is specimen size independent, even when nominal stress was well beyond general yield and the crack was propagating beyond maximum load. Using this observation, it is shown how instability can ge predicted under conditions ranging from elastic-plastic to fully plastic. This establishes the ground work for future studies on how R-curve is affected by specimen type or by stress triaxiality conditions.