Q:

Table with different column widths

I wish to create a table that has different row column size. How can i achieve something like that? 
EDIT #1:
I tried
\begin{tabular}{|p{4cm}|p{2cm}|p{2cm}|p{1cm}|p{2cm}|p{3cm}|p{2cm}|} \hline
    \multicolumn{4}{c}{imm[31:12]}  &   rd  & 0110111 & TEST \\\hline

\end{tabular}

And the result is this

A:

How about this? I try to reproduce the first five rows of your table.
\documentclass{article}
\usepackage{array}
% https://tex.stackexchange.com/a/12712/156344
\newcolumntype{C}[1]{>{\centering\let\newline\\\arraybackslash\hspace{0pt}}m{#1}}
\begin{document}
\begin{table}
\caption{Some caption for the table}
\begin{tabular}{|C{2cm}|C{1cm}|C{1cm}|C{1cm}|C{2cm}|C{1.5cm}|} \hline
    \multicolumn{4}{|c|}{imm[31:12]} & rd & 0110111 \\\hline
    \multicolumn{4}{|c|}{imm[31:12]} & rd & 0010111 \\\hline
    \multicolumn{4}{|c|}{imm[20\textbar10:1\textbar11\textbar19:12]} & rd & 1101111 \\\hline
    \multicolumn{2}{|c|}{imm[11:0]} & rs1 & 000 & rd & 1100111 \\\hline
    imm[12\textbar10:5] & rs2 & rs1 & 000 & imm[4:1\textbar11] & 1100011\\\hline
\end{tabular}
\end{table}
\end{document}

A:

with use of the makecell package for more vertical space around cells' contents, defining two new commands for shorter table's code:
\documentclass{article}
\usepackage{array, makecell}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\newcommand\mcF[1]{\multicolumn{4}{|c|}{#1}}
\newcommand\mcT[1]{\multicolumn{2}{|c|}{#1}}
\usepackage[skip=1ex]{caption}

\begin{document}
    \begin{table}
    \setcellgapes{3pt}
    \makegapedcells
    \centering
\caption{Some caption for the table}
\begin{tabular}{|C{20mm}|*{3}{C{13mm}|}*{2}{C{20mm}|}}
    \hline
\mcF{imm[31:12]}                            & rd                    & 0110111   \\  \hline
\mcF{imm[31:12]}                            & rd                    & 0010111   \\  \hline
\mcT{imm[11:0]}             & rs1   & 000   & rd                    & 1100111   \\  \hline
imm[12\textbar10:5] & rs2   & rs1   & 000   & imm[4:1\textbar11]    & 1100011   \\  \hline
0000000             & shamt & rs1   & 001   & rd                    & 0010011   \\
    \hline
\end{tabular}
    \end{table}
\end{document}

