This paper presents the spatial variation of area-orientated annual maximum daily rainfall (AMDR), represented by well-fitted generalized extreme value (GEV) distributions, from two century-long datasets of Great Britain (GB) and Australia with respect to three spatial properties: geographic locations, sizes and shapes of the region of interest (ROI). The results show that the spatial variation of GEV location-scale parameters are dominated by geographic locations and area sizes. In GB, there is an eastward-decreasing banded pattern compared with a concentrically-increasing pattern from the middle to coasts in Australia. The parameters tend to decrease with increased area sizes in both studied regions. Although the impact of the ROI shapes is insignificant, the round-shaped regions usually have higher-valued parameters than the elongated ones. The findings provide a new perspective to understanding the heterogeneity of extreme rainfall distribution over space driven by the complex interactions among climate, geographical features, and the practical sampling approaches.