The microhardness indentation load/size effect (ISE) on the Knoop microhardness of single crystals of TiO2 and SnO2 has been investigated. Experimental results have been analysed using the classical power law approach and from an effective indentation test load viewpoint. The Hays/Kendall concept of a critical applied test load for the initiation of plastic deformation was considered, but rejected to explain the ISE. A proportional specimen resistance (PSR) model has been proposed that consists of the elastic resistance of the test specimen and frictional effects at the indentor facet/specimen interface during microindentation. The microhardness test load, P, and the resulting indentation size, d, have been found to follow the relationship $$P = a_1 d + a_2 d^2 = a_1 d + (P_c /d_0^2 ) d^2$$ The ISE is a consequence of the indentation-size proportional resistance of the test specimen as described by a 1. a 2 is found to be related to the load-independent indentation hardness. It consists of the critical indentation load, P c, and the characteristic indentation size, d o.