Assume the "Big Splash" or "Giant Impact" (whichever name you prefer) theory is more or less correct.
EDIT: With so many variations of this theory, with so many names and qualifiers on them, such as "direct impact theory" and "glancing blow hypothesis", etc., let's assume that the impact was direct enough that nothing of Theia (other than Earth and the Moon) was left that was large enough to for its gravity to compress it into a hydrostatically equilibrious shape, and scatters too much to coalesce (on its own) in to anything large enough either.
Assume "Theia" was the size of Mars (some variations on the theory use other sizes, but for the sake of this question, let' stick with Mars)
How big was Earth before the impact?
It can't be as simple as "Earth + Moon - Theia (Mars) = pre-impact size", can it? Because there must have been some debris ejected completely from the earth+moon system, right? or was it such a small amount that it can be considered negligible?