Here, we present the synthesis of graphene with controlled lateral size and thickness via epitaxial growth on Ru(0001).Either ethylene (C2H4) or coronene (C24H12) was used as the precursors for graphene growth.The mechanism of how 2D graphene overlayers evolve from the hydrocarbon precursors was investigated by high resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM) and photoemission electron microscopy (PEEM).We found that identical-size graphene nanoclusters with the 3-C6 ring (C24 cluster) and 7-C6 ring (C13 cluster) structures form on Ru(0001) after pyrolysis of both C2H4 and C24H12 at elevated temperatures (around 900 K).