Best-fit parameters of the star-forming size–mass relation: intercept in kpc at 10⁹ M_⊙ (left), slope α (centre), and intrinsic scatter σ_logREFF (right) as functions of rest-frame wavelength (using median redshifts per bin; see Table A.1). The 6 ≤ z < 9 slope is omitted from the centre panel as it was fixed during fitting (see Section 3.4). The observed decrease in slope with wavelength aligns with colour gradients, while the large σ_logREFF at λ < 0.5 μm may reflect off-centre bursty star formation in z > 3 star-forming galaxies.