Effective size as a function of age or number of SNe, respectively, extrapolated from the model N 1 x 10 and the mean of the momentum constraint Eq. (2) for various values of α; namely, R_α=R_N 1 × 10(t=t_age) ×[Ṅ_SN, LB(t_age)/1 Myr⁻¹]^α, where Ṅ_SN, LB= N_SN, LB/t_age. For α ≲ 0.225, the extrapolated sizes match the LB for ages in the range 3.5−5.5 Myr, while for α ≳ 0.225 the two constraints on the size and momentum cannot be satisfied simultaneously.