This paper examines the relationship between the government size and economic growth and determines the optimal level of government size in OPEC countries over 1960-2015. In this regards, we use a large dataset in a non-linear panel Generalized Method of Moments framework. We show that this relationship is statistically significant above and below the optimal level. In this study, the government expenditure (percentage of GDP) is defined as an index for the government size. According to the findings, the threshold level of optimal government size is 19 percent for the countries. The results show that the government size has a positive impact on economic growth in the first regime, while in the second regime, this is negative. In other words, the results confirm the Army curve hypothesis. On the other hand, the average size of government among all the countries is 24.7 percent, which represents government spending is putting on the slopes of diminishing returns. Therefore, the government size should be reduced to achieve their optimal amount.