The flow of frictionless granular particles is studied with stress-controlled discrete element modeling simulations for systems varying in size from 300 to 100,000 particles. The volume fraction and shear stress ratio µ are relatively insensitive to system size fo a wide range of inertial numbers I. Second-order effects in strain rate, such as second normal stress differences, require large system sizes to accurately extract meaningful results, notably a non-monotonic dependence in the first normal stress difference with strain rate. The first-order rheological response represented by the µ(I) relationship works well at describing the lower-order aspects of the rheology, except near the quasi-static limit of these stress-controlled flows. The pressure is varied over five decades, and a pressure dependence of the coordination number is observed, which is not captured by the inertial number. Large fluctuations observed for small systems N ≤ 1,000 near the quasi-static limit can lead to arrest of flow resulting in challenges to fitting the data to rheological relationships. The inertial number is also insufficient for capturing the pressure-dependent behavior of property fluctuations. Fluctuations in the flow and microstructural properties are measured in both the quasi-static and inertial regimes, including shear stress, pressure, strain rate, normal stress differences, volume fraction, coordination number and contact fabric anisotropy. The fluctuations in flow properties scale self-similarly with pressure and system size. A transition in the scaling of fluctuations of stress properties and contact fabric anisotropy are measured and proposed as a quantitative identification of the transition from inertial to quasi-static flow.