Previous studies investigated the ability of Nb–B inoculants to refine grain in Al alloys; however, their dendritic network was not considered. In the present study, the synergistic effect of 0.5Nb-0.1B wt.% on the dendritic microstructure scale and grain size (GS) of a 6201-base alloy with an extensive range of solidification cooling rates is investigated. It is demonstrated that 0.5Nb-0.1B wt.% refined GS for either columnar or equiaxed morphology independent of the cooling rate, with a refining capability of 13-fold reduction in GS. The secondary dendritic spacing (λ2) also decreased, specifically for low cooling rates, as compared to the non-modified alloy. It is proposed that the GS to λ2 ratio may characterize α-Al cell micromorphology in the 6201 + NbB alloy, which occurs when this ratio is lower than 6. High-cooling-rate cellular arrays were observed for cooling rates higher than 5.5 °C/s. Although a low GS and λ2 result in an increased strength of the 6201 + NbB alloy, a limit for optimizing this property via grain refining appears to be reached at approximately 190 MPa for the GS ranging from 142 to 239 μm and the λ2 ranging from 22.6 to 24.5 μm.