Biological soil crusts (biocrusts) are mixed communities of cyanobacteria, lichens and mosses in different ratios, contributing to important ecological functions in arid and semiarid regions worldwide. Biocrusts are spatially variable, and the variability in biocrust composition and coverage is scale-dependent. The following question can then be asked: What is the appropriate spatial scale for observing ecological functions? Without clarifying this issue, we cannot fully understand the ecological functions of biocrusts. The key to answering this question is to determine a threshold area, or representative elementary area (REA). Accordingly, we analyzed red–green–blue (RGB) images of 90 biocrust plots (2.0 m × 2.0 m) from nine revegetated grasslands in the Hilly Loess Plateau region of China. The variability in biocrust composition and coverage across the plot sizes was studied by gradually expanding the plot size from 0.01 m² to 4.00 m². The results showed that as the plot sizes increased from 0.01 m² to 4.00 m², the number of biocrust types logarithmically increased. Biocrust patches of a particular type (such as moss, cyanobacteria or lichen) were often characteristic of a plot size of 0.01 m², whereas plot sizes larger than 0.25 m² supported mixed biocrusts of multiple patch types. The variability in coverage of mixed biocrusts logarithmically decreased with increasing plot sizes. The coverage of mixed biocrusts maintained an approximately constant value after a certain critical plot size was reached (1.00 m² in this study). Our data indicated that REAs of mixed biocrusts exist at the slope scale. The REAs of mixed biocrusts were 0.5–1.0 m², 1.5–2.5 m² and 3.0–3.8 m², with alpha (α) values of 0.1, 0.05 and 0.01, respectively. The size of the REAs on the north-facing slope was larger than that on the south-facing slope, and the patch density of biocrusts had an important influence on the REAs. The results of this study could provide a method for determining the REAs of mixed biocrusts and guide surveys and experimental layouts.