With the potential for an impact event to trigger volcanic activity on the opposite side of the planet, what is the largest plateau of flood basalt physically capable of forming on Earth, post-impact?
Well, the maximum possible would be about 510 million square kilometres, eg. the surface area of the earth:
The hypothesised Theia impact would have probably resurfaced much of the earth.
The largest flood basalt actually found would be the Siberian Traps, a mere 7 million square kilometres. The problem is that no-one is really sure quite why large igneous provinces form, so there isn't really any useful theoretical work on their potential scale. The Siberian Traps look to be the largest that formed in the last 500 million years or so, and may or may not have been triggered by an impact event (the cause of the Permian-Triassic extinction event hasn't been pinned down as yet). The fact that the traps are substantially larger than any others suggests that they represent an upper limit of possible flood basalt sizes... this of course isn't guaranteed to be true, but it does seem relatively plausible.