The aim of this study is to present a representative volume element for nanocomposites with different microstructural features using stochastic finite element approach. To that end, computer simulated microstructures of nanocomposites were generated to include a variety of uncertainty present in geometry, orientation and distribution of Carbon Nanotubes. Microstructures were converted into Finite Element Models based on an image based approach for determination of elastic properties. For each microstructure type, 50 realization of synthetic microstructures were generated to capture the variability as well as the average values. Computer simulated microstructures were generated at different length scales to determine the change in mechanical properties as function of length scale. A representative volume element is defined at a length scale beyond which no change in variability is observed. The results show that there is no universal RVE applicable to all properties and microstructures, however, the RVE size is highly dependent on microstructural features. Microstructures with agglomeration tend to require larger RVE. Similarly, random microstructures require larger RVE compared to aligned microstructures.