Graphite nanoplatelets (GNPs) are outstanding thermal conductive fillers due to their unique crystal structure which consists of layered graphene formed by sp(2)-hybridized carbon atoms, but their preparation process is time-consuming. Herein, a high-efficiency and low-cost method is developed to prepare GNPs with controlled size without destroying their crystal structure. GNPs with an average size around 20m are obtained by a facile nitric acid treatment within 0.5h, and 1-5m GNPs are produced by controlling ultrasonic time within 6h in cheap commercial detergent. In addition, thermal network is fabricated by combining similar to 20m GNPs with 1-5m GNPs in epoxy. The existing researches have qualitatively demonstrated that hybrid carbon fillers with obvious size differences can enhance the thermal conductivity of epoxy through size synergistic effects, but rare quantitative studies pay attention to optimize size synergistic effects. This work demonstrates that different degrees of size synergistic effects can be achieved by tuning the size of small-sized GNPs, the proportion of different sized GNPs and the content of hybrid fillers. The results show that when the hybrid filler loading is 20wt% and the ratio of similar to 20m GNPs to 1.82m GNPs is 17:3, the composite produces the optimal size synergistic effect and thermal conductivity reaches 1.33W/mK, increased by 739% over neat epoxy. This composite has promising application in the thermal management areas due to its high thermal conductivity and economic competitiveness.