  Cosmic 21cm interferometry is set to revolutionize our understanding of the
Epoch of Reionization (EoR) and the Cosmic Dawn (CD). However, the signal has
structure on a huge range of scales, requiring large simulation boxes to
statistically capture the relevant fields. In this work we quantify the minimum
box size for simulating the power spectrum (PS) of the cosmic 21cm signal. We
perform multiple realizations of the initial conditions, for a range of box
sizes. We quantify convergence with respect to a mock observation of box length
1.1 Gpc, with thermal noise computed for a 1000h observation with SKA1-low. We
find that simulations of box lengths 200-300 Mpc underestimate the large-scale
power during the CD by ~7-9 % on average. We conclude that box lengths of
$L\gtrsim250$ Mpc are needed to converge at the level of <~1$\sigma$ of the
total noise.
