In this paper, we propose an analytical model for dimensioning of Orthogonal Frequency Division Multiple Access (OFDMA) systems in 5G networks by considering Internet of Thing (IoT) application using stochastic geometry. In these systems, some communication is lost when the number of required subcarriers is greater than the number of the available subcarriers. We compute the upper bound of the lost communication probability for downlink. In such a system, the position of the receiving users is modeled by the Poisson point process (PPP). The number of subcarriers dedicated to each user depends on its Signal to Noise Ratio (SNR), position and the shadowing, hence for calculating the number of subcarriers, it is needed to use stochastic geometry. Since the focus of our work is on IoT application in 5G networks, a multi-group user system with each group of users having its own application and throughput requirement is considered. For having dimensioning in terms of subcarriers, we present concentration inequality for functions defined on PPP to calculate the upper bound of loss probability. The performance of the upper bound in different range of user intensity is investigated.