  Preparing for the first detection of the cosmic 21-cm signal from large-scale
interferometer experiments requires rigorous testing of the data analysis and
reduction pipelines. To validate that these pipelines do not erroneously remove
or add features that can mimic the cosmic signal (e.g. from side-lobes or
large-scale power leakage), we require reionisation simulations larger than the
experiments primary field of view. For an experiment such as the MWA, with a
field of view of $\sim25^{2}$ deg.$^{2}$, this would require a simulation of
several Gpcs, which is currently infeasible. To overcome this, we developed a
simplified version of the semi-numerical reionisation simulation code 21CMFAST
preferencing large volumes over some physical accuracy by assuming linear
theory for structure formation. With this, we constructed a 7.5 Gpc comoving
volume with voxel resolution of $\sim1.17$ cMpc tailored specifically to the
binned spectral resolution of the MWA. This simulation was used for validating
the pipelines for the 2020 MWA 21-cm power spectrum (PS) upper limits (Trott et
al.). We then use this large-volume simulation to explore: (i) whether smaller
volume simulations are biased by the missing large-scale modes, (ii)
non-Gaussianity in estimates of the cosmic variance, (iii) biases in the
recovered 21-cm PS following foreground wedge removal and (iv) the impact of
tiling smaller volume simulations to achieve extremely large volumes. In
summary, we find: (i) no biases from missing large-scale power, (ii)
significant contribution from non-Gaussianity in the cosmic variance as
expected following Mondal et al. (iii) an over-estimate of the 21-cm PS of
10-20 per cent following wedge mode excision for our particular model and (iv)
tiling smaller volume simulations under-estimates the large-scale power and
also the estimated cosmic variance.
