Short answer, there are various estimates, with large uncertainties, ranging from 4 to 7 million km2.
Pulling from the link provided by Keith McClary in the comments:
Estimating the original extent of the Traps is difficult, given the
likely erosion over the last 250 Ma. This is especially important for
the relatively easily eroded pyroclastic deposits. Whilst preservation
may have been improved by burial in the West Siberian Basin and
Yenesei-Khatanga Trough, it is unclear how much tectonic uplift and
erosion occurred in these basins during the early Triassic, prior to
burial. Milanovskiy (1976) estimated that the original extent of the
Traps was about 4x106 km2, but there must be considerable latitude in
this figure. Masaitis (1983), for example, has suggested that the
Traps originally extended over a region of ~7 million km2.
Masaitis, V.L., 1983. Permian and Triassic volcanism of Siberia: Problems of dynamic reconstructions. Mineralogicheskogo Obshchestva, 112(4): 412-425.
Milanovskiy, Y.Y., 1976. Rift zones of the geologic past and their associated formations. International Geology Review, 18: 619-639.
Additionally I have found another estimate Saunders (2009):
The original total area (and volume) of the Siberian flood basalt
province is difficult to estimate, because a substantial portion is
covered by thick sedimentary se- quences and because an unknown amount
has been re- moved by erosion. Taking the limits of the existing out-
crops as a maximum extent, then we estimate that an area of
approximately 5×106 km2 of Siberia may have been partly covered by
basalt1. It is unlikely that an accurate figure of area (and volume)
will ever be known.
Saunders, A., Reichow, M. The Siberian Traps and the End-Permian mass extinction: a critical review. Chin. Sci. Bull. 54, 20–37 (2009). https://doi.org/10.1007/s11434-008-0543-7