Platooning has emerged to be one of the most promising applications for connected and automated vehicles (CAVs). However, there is still limited research on the effect of platooning configurations. This study sets out to investigate the effect of CAV platoon configurations at a typical isolated roundabout in a mixed traffic environment. Investigated platoon configurations include maximum platoon size, platoon willingness, and platoon type. Extensive simulation experiments are carried out in simulation of urban mobility (SUMO), considering various traffic conditions, including different penetration rates, traffic flows, and turning percentages. Results show that: (1) increasing the maximum platoon size and platoon willingness generally improves the throughput increment and delay reduction; and (2) heterogeneous platoons outperform homogeneous platoons in all traffic conditions.