> arbitrarily large

Space is not empty. Satellites have to be boosted all the time because of drag. Massive panels would only worsen that. Once you boosters are empty the satellite is toast.

the point wasn't that a 1 m^2 solar panel could theoretically be kept reasonably cool at the cost of a miles long radiator... nono, the point was that you could attain any desirable temperature this way, arbitrarily close to 4K.

for a reasonable temperature (check my comment for updated calculations) the height of a square based pyramidal satellite would be about 3 times the side length of its base, quite reasonable indeed. Thats with the square base of the pyramid as solar panel facing the sun, and the top of the pyramid facing away, so all sides are in the shade of the base. I even halved my theoretical cooling power to keep calculations simple: to avoid a long confusing calculation of the heat emitted by earth, I handicapped my design so 2 of the pyramidal side surfaces are reflective (facing earth) and the remaining 2 side triangles of the pyramid are the only used thermal radiative cooling surfaces. Less pessimistic approaches are possible, but would make the calculation less didactic for the HN crowd.

"Satellites have to be boosted all the time because of drag."

On Low Earth Orbits (LEOs), sure, but the traces of atmosphere that cause the drag disappear quite fast with increasing altitude. At 1000 km, you will stay up for decades.