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Physical size dependence of reaction energetics of CO desorption from graphene ribbons was investigated using Density Functional Theory. Using the previously reported 3-step mechanism for CO desorption from a char model, the results revealed several important size dependences. For the ground state, increasing the ribbon length had small effects on the energy profile of the CO desorption. However, the energy gap between the electronic low spin ground state and its high spin excited state at stationary points along the CO desorption reaction coordinate decreased to thermally accessible range as the ribbon size increases. This suggested contribution of excited states in the chemistry of graphene production process that involved in CO desorption would be important. The results also indicated that the effects of physical size on the relative barriers and stabilities of reaction intermediates along the CO desorption channel could be attributed to the disruption of π-conjugation of the graphene ribbon surface.