Matching delimiter sizes in a split equation

Question

When writing long equations using amstex split, is there a way to match delimiter sizes from one line to the next? Since we're "forced" to close them before the line breaks (using \right.), I'm not sure I understand how LaTeX would know from one line to the next which size to use (hence the difference in sizes).

In other words, in this equation:

\begin{equation}
\begin{split}
E_z = \frac{1}{4 \pi \epsilon _0} \int _{-h_-} ^{h_+} \left[ \frac{2 \left( z - z' \right) ^2 - r^2}{R^5} \int _{-\infty} ^t  I \left( z' , \dfrac{\tau - R}{c} \right) {\rm d} \tau \right. \\
\left. \frac{2 \left( z - z' \right) ^2 - r^2}{cR^4} I \left( z' , \dfrac{\tau - R}{c} \right) - \frac{r^2}{c^2R^3} \frac{\partial I I \left( z' , \dfrac{\tau - R}{c} \right)}{\partial t} \right] {\rm d} z'
\end{split}
\end{equation}

the square brackets on the first line are considerably smaller than the square brackets on the second line.