Relation between stellar surface density and size (defined as half-mass or half-light radius) in our simulations and as observed in local star clusters. The black solid squares, dark-cyan solid squares, red solid circles and light-cyan diamonds are the star clusters or clumps retrieved at z = 10.5 in the FC22, ‘Winds, SFE=0.1’, ‘Winds, SFE=1.0’ and ‘Winds, THIMF’ model, respectively, and the small black dots are the observational dataset of young star clusters from the Legacy Extragalactic UV Survey (Ryon et al. 2017; Brown & Gnedin 2021). The thick red and thin black line is a linear fit (in log(∑_*) - log(R_half) space) to the relation found in the ‘Winds, SFE=1.0’ model and in the observational dataset, respectively. Each grey dashed line represents the relation between Σ_* and R_half at fixed stellar mass, for which we have considered various values between 10² M_ʘ and 10⁶ M_ʘ.