This paper explores the possibility of a large internal dimension at low energies, around a few TeV, which is a prediction of perturbative string theories. The size of this dimension is related to the scale of supersymmetry breaking. The paper discusses how this scenario is consistent with perturbative unification up to the Planck scale in certain four-dimensional string models and has significant phenomenological consequences. It also addresses the challenge of spontaneous supersymmetry breaking in string theories, which is difficult to achieve in perturbation theory. The paper suggests that a small supersymmetry breaking scale can be achieved through discrete parameters or by relating it to an internal compactification radius. It also discusses non-perturbative effects, such as gaugino condensation, which can lead to a large supersymmetry breaking scale. However, the paper argues that in certain string models, the supersymmetry breaking can be achieved without large corrections to the coupling constants. The paper also discusses the implications of having a large internal dimension, including the repetition of states with the same quantum numbers and the appearance of chirality after compactification. It concludes that a large internal dimension at a few TeV is consistent with experimental data and implies a spectrum of superpartners of known particles near the TeV scale. The paper also highlights the need for further analysis of the cosmological constant problem and the determination of the size of the extra dimension.This paper explores the possibility of a large internal dimension at low energies, around a few TeV, which is a prediction of perturbative string theories. The size of this dimension is related to the scale of supersymmetry breaking. The paper discusses how this scenario is consistent with perturbative unification up to the Planck scale in certain four-dimensional string models and has significant phenomenological consequences. It also addresses the challenge of spontaneous supersymmetry breaking in string theories, which is difficult to achieve in perturbation theory. The paper suggests that a small supersymmetry breaking scale can be achieved through discrete parameters or by relating it to an internal compactification radius. It also discusses non-perturbative effects, such as gaugino condensation, which can lead to a large supersymmetry breaking scale. However, the paper argues that in certain string models, the supersymmetry breaking can be achieved without large corrections to the coupling constants. The paper also discusses the implications of having a large internal dimension, including the repetition of states with the same quantum numbers and the appearance of chirality after compactification. It concludes that a large internal dimension at a few TeV is consistent with experimental data and implies a spectrum of superpartners of known particles near the TeV scale. The paper also highlights the need for further analysis of the cosmological constant problem and the determination of the size of the extra dimension.