Coarse-grained soil (CGS) is a vital geotechnical material extensively used in engineering projects. Scale effects pose challenges to accurately assessing its shear strength, and the influences of scaling methods and moisture content remain underexplored. This study investigates the effects of maximum particle size ($d_{\max }$), scaling techniques, and moisture content on the shear strength of CGS through large-scale direct shear tests and in situ direct shear tests. CGS samples from the Jilangtan platform in Qinghai Province, originally with a $d_{\max }$ of 60 mm, were scaled to 30 mm using the Equivalent Substitution Method (ESM) and Similar Gradation Method (SGM). Tests were conducted at moisture contents of 0.91% (air-dried), 2.45% (natural), and 7.66% (saturated). Results show that ESM consistently yields higher shear strength than SGM across all moisture levels. Scale effects are minimal under air-dried conditions but become pronounced at higher moisture contents. While all large-scale laboratory test results deviate somewhat from in situ findings, shear strength parameters derived from SGM align more closely with field observations.