When a physicist says arbitrarily large it could even be in a dimensionless sense. It doesn't matter how small or large the solar panel is:
for a 4 m x 4 m solar panel, the height of the pyramid would have to be 12 m to attain ~ 300 K on the radiator panels. Thats also the cold side for your compute.
for a 4 km x 4 km solar panel the height of the pyramid would be 12 km.