What was earth's mass before the collision with theia? 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?
I'm not sure if there was a significant amount of debris, however the calculation you mentioned would at least give an upper bound on the value:

$
M_{piE}=\text{Greatest possible mass of pre-impact Earth}\\
M_E=\text{Mass of Earth today}=6.0\times10^{24}kg\\
M_M=\text{Mass of the Moon}=7.3\times10^{22}kg\\
M_T=\text{Mass of Theia}\approxeq6.4\times10^{23}kg\\
$
$$
\begin{eqnarray*}
M_{piE}&amp;=&amp;M_E+M_M-M_T\\
&amp;=&amp;(6.0\times10^{24})+(7.3\times10^{22})-(6.4\times10^{23})\\
&amp;=&amp;5.4\times10^{24}kg
\end{eqnarray*}
$$
So it wouldn't have been more than about $5.4\times10^{24}kg$.