Different storage capacities and various file sizes are two crucial factors which impact the effectiveness of cache placement in device-to-device networks. However, most of existing literature assumes all users have homogeneous cache capability and all involved file sizes are identical. In this paper, files are placed into groups according to their different sizes and the storage capacities are various among all the involved user devices. Considering these two vital factors, group caching scheme (GCS) algorithm for the optimal cache strategy is investigated by formulating an optimization problem to maximize the cache hit probability solved by using Karush-Kuhn-Tucker conditions. Furthermore, random linear network coding-based GCS is developed to eliminate the negative impact of file size to cache hit probability due to the observation that the probability of small size files being cached may be higher than that of large size but more popular files. Finally, simulation results show the effectiveness of the proposal which outperforms the existing schemes.