How can I insert this table?

Question

I have the table of this code. When I insert this table, the text is very small and I dont see a lot. How can I insert this table for a better comprehension of the text?

\begin{table}[h]
\begin{center}
\begin{tabular}{ | l | c | c | c | c | c | c | c | c | c | c | c | c | c | c | c | c | }
\hline
\multirow{2}{*}{\textbf{Method Function}} & \multirow{2}{*}{Lineal} & \multicolumn{4}{c|}{Pol} &     \multicolumn{5}{c|}{Gaussian} & \multirow{2}{*}{S method} & \multirow{2}{*}{Line} & \multirow{2}{*}{Upload} & \multirow{2}{*}{Fourier} & \multirow{2}{*}{Marconi} & \multirow{2}{*}{Tesla} \\
\cline{3-11}
& & grade 1 & grade 2 & grade 3 & grade 4 & $\sigma=0.15$ & $\sigma=0.25$ & $\sigma=0.5$ & $\sigma=2$ & $\sigma=3$ & & & & & &\\
\hline
Template & 57\% & 80\% & 92 & 89 & 89 & 94 & 97 & 97 & 94 & 92 & a & b & c & d & e & f \\
\hline
Template & 57\% & 80\% & 92 & 89 & 89 & 94 & 97 & 97 & 94 & 92 & a & b & c & d & e & f \\
\hline
\end{tabular}
\end{center}
\end{table}

Answer 1

Your table is to wide to be fitted even on A3 size paper ... or rotated on A4 size paper. You had to find a way to redesign it. One attempt, how to do this is below:

\documentclass{article}
    \usepackage{multirow}
    \usepackage[margin=25mm,showframe]{geometry}

\begin{document}
    \begin{table}[h]
\centering
\small
\setlength\tabcolsep{3.6pt}
\begin{tabular}{ | l | *{16}{c |}}
\hline
\multirow{2}{14mm}{\textbf{Method\newline Function}} & \multirow{2}{*}{Lineal} & \multicolumn{4}{c|}{Pol, for grade} &     \multicolumn{5}{c|}{Gaussian, for deviation $\sigma$} & \multirow{2}{12mm}{\centering S\\ method} & \multirow{2}{*}{Line} & \multirow{2}{*}{Upload} & \multirow{2}{*}{Fourier} & \multirow{2}{*}{Marconi} & \multirow{2}{*}{Tesla} \\
\cline{3-11}
& & 1 & 2 & 3 & 4 & $0.15$ & $0.25$ & $0.5$ & $2$ & $3$ & & & & & &\\
\hline
Template & 57\% & 80\% & 92 & 89 & 89 & 94 & 97 & 97 & 94 & 92 & a & b & c & d & e & f \\
\hline
Template & 57\% & 80\% & 92 & 89 & 89 & 94 & 97 & 97 & 94 & 92 & a & b & c & d & e & f \\
\hline
\end{tabular}
    \end{table}
\end{document}