About big parenthesis larger than Bigg

The ams parenthesis of specific size are typeset by using \left and \right and putting in an invisible "thing" of a certain height to ensure that they are big enough. Mimicking that, you could do: \left(\rule{0cm}{2cm}\right. to get a left parenthesis of 2cm height. Note that the AMS command also adds a little horizontal space which you might need to put in as well. If you do this often, you could easily define new sizes:

\makeatletter
\newcommand{\vast}{\bBigg@{4}}
\newcommand{\Vast}{\bBigg@{5}}
\makeatother

Then in your document:

\[
\Vast( \frac{\prod_0^\infty k}{\sum_0^\infty n} \Vast)
\]

produces:

I recommend also taking a look at Vertically asymmetric size variation for parentheses about vertically centring parentheses.

(Note I've assumed that you're using amsmath since you tag it with amsmath and the commands \big and so forth are ones that amsmath produces. If you're using a different package to get these commands, then this solution won't work.)

I had the same requirement as Anand and came across this page. The solution recommended by Loop Space works flawlessly. However, when the equations are already too complex, typing \left(\rule{0cm}{2cm}\right. inside the math environment adds to the clutter. Therefore, I declared the following commands in my preamble and call them with the required height where it is necessary.

\documentclass{article}
\usepackage[cmex10]{amsmath}

\newcommand{\prb}[1]{\mathop{\mathrm{Pr}}\nolimits \left\{ {#1} \right\} }

\newcommand{\cstpl}[1]{\left(\rule{0 cm}{#1}\right.}
\newcommand{\cstpr}[1]{\left.\rule{0 cm}{#1}\right)}

\begin{document}

\begin{align*}
& Q = \cstpl{0.8cm} \prb{\frac{A^2_i}{B+C^3_i} \ge D } - \\
& \prb{\frac{E^2_j}{F^2_k+G} \ge H } \frac{ L }{ M } \cstpr{0.8cm}.
\end{align*}

\end{document}