Rather than try to reduce the size of the equation in order to make it fit on a single line, I would employ a multline
environment and typeset the equation across four lines: Three lines for the numerator, and the fourth and final line for the denominator. Note that in the following solution, I've dispensed with \frac
notation.

\documentclass{article}
\usepackage{amsmath} % for 'multline' env.
\begin{document}
\begin{multline}\label{EEDIeq}
\Biggl\{\biggl(\,\prod_{j=1}^n f_j\biggr)
\biggl(\,\sum_{i=1}^{n_{\mathrm{\mathrm{ME}}}}
P_{\mathrm{\mathrm{ME}}(i)}\cdot
C_{\mathrm{FME}(i)}\cdot
\mathit{SFC}_{\mathrm{\mathrm{ME}}(i)}\biggr)
+ P_{\mathrm{AE}}\cdot C_{\mathrm{FAE}}
\cdot \mathit{SFC}_{\!\mathrm{AE}}\\
+ \biggl[\biggl(\,\prod_{j=1}^n f_j\cdot
\sum_{i=1}^{n_{\mathrm{PTI}}} P_{\mathrm{PTI}(i)}
-\sum_{i=1}^{n_{\mathrm{eff}}}f_{\mathrm{eff}(i)}\cdot
P_{\mathrm{AE}_{\mathrm{eff}}(i)}\biggr)
C_{\mathrm{FAE}}\cdot \mathit{SFC}_{\!\mathrm{AE}}\biggr] \\
-\biggl(\,\sum_{i=1}^{n_{\mathrm{eff}}}f_{\mathrm{eff}(i)}
\cdot P_{\mathrm{eff}(i)}\cdot C_{\mathrm{FME}}
\cdot \mathit{SFC}_{\mathrm{ME}}\biggr) \Biggr\}\\
\Big/\Bigl\{f_i\cdot f_c\cdot f_l\cdot \mathit{Capacity}
\cdot f_w\cdot V_{\mathrm{ref}}\Bigr\}
\end{multline}
\end{document}