my matrix doesnt fit the page

Use symbols:

\documentclass[11pt]{article}
\usepackage{amsmath}

\DeclareMathOperator{\diag}{diag}

\begin{document}

\begin{equation*}
[a_{ii}]=[\phi]^T[A][\phi] = \diag(\delta_1,\delta_2,\delta_3,\delta_4,\delta_5,\delta_6)
\end{equation*}
where
\begin{equation*}
\begin{aligned}
\delta_1 &= -0.0829 + 0.2686i, \\
\delta_2 &= -0.0829 - 0.2686i, \\
\delta_3 &= -0.2040 + 2.5845i, \\
\delta_4 &= -0.2040 - 2.5845i, \\
\delta_5 &= 3.2154, \\
\delta_6 &= -3.0137
\end{aligned}
\end{equation*}

\end{document}

You can also type in the full matrix, but the “diag” notation is clearer, in my opinion.

Yet another possibility is negative space as column separator.

\documentclass[11pt]{article}
\usepackage{amsmath}
\begin{document}
\newcommand\shrink{\hspace{-3em}}
\begin{align*}
[a_{ii}]&=[\phi]^T[A][\phi]\\
&=
\left[\begin{array}{@{}c@{\shrink}c@{\shrink}c@{\shrink}c@{}c@{\hspace{-0.5em}}c@{}}
-0.0829 + 0.2686i&0&0&0&0&0\\
0&-0.0829 - 0.2686i&0&0&0&0\\
0&0&-0.2040 + 2.5845i&0&0&0\\
0&0&0&-0.2040 - 2.5845i&0&0\\
0&0&0&0&3.2154&0\\
0&0&0&0&0&-3.0137
\end{array}
\right]
\end{align*}
\end{document}

Yet another option, change the textsize:

\documentclass[11pt]{article}
\usepackage[fleqn]{amsmath}

\begin{document}

\begin{tiny}

{\setlength{\mathindent}{0cm}
\begin{equation*}
[a_{ii}]=[\phi]^T[A][\phi]=\begin{bmatrix}
-0.0829 + 0.2686i&0&0&0&0&0\\
0&-0.0829 - 0.2686i&0&0&0&0\\
0&0&-0.2040 + 2.5845i&0&0&0\\
0&0&0&-0.2040 - 2.5845i&0&0\\
0&0&0&0&3.2154&0\\
0&0&0&0&0&-3.0137
\end{bmatrix}
\end{equation*}
}

\end{tiny}

\end{document}