[Tex/LaTex] How to increase the height of a single row in table

Question

As seen in the figure, the table is incomplete and extends beyond the page. I need to fit it within the page. So, I need to increase row height and decrease column width. How can I do this? Also, the text is not aligned to the centre. I don't understand the problem here.

\begin{table}[ht]
\begin{center}
\begin{tabular}{||c||c|c||c|c|c|c||} 
\hline
\multirow{2}{2cm}{\textbf{Atlas Resolution}}  & \multicolumn{2}{p{3cm}||}{\centering \textbf{Variant-1}} & \multicolumn{2}{p{3cm}||}{\centering    \textbf{Variant-2}} & \multicolumn{2}{p{3cm}||}{\centering \textbf{Variant-3}} \\ \cline{2-7}
& \textbf{Average Render Time (ms)} & \textbf{IFPS} & \textbf{Average Render Time (ms)} & \textbf{IFPS} & \textbf{Average Render Time (ms)} & \textbf{IFPS}\\ 
\hline\hline
512$\times$512 &  &  &  &  &  &  \\ 
\hline
1024$\times$1024 &  &  &  &  &  &  \\ 
\hline
18432$\times$18432 &  &  &  &  &  &  \\ 
\hline
19456$\times$19456 &  &  &  &  &  &  \\  [1ex]
\hline
\end{tabular}
\caption{List of sizes for various resolution of Dataset-I, compressed variant Dataset-II and different slices variant Dataset-II}
\end{center}
\end{table}
\label{tab:1}

Updated table:

Answer 1

See, it the following work around is acceptable to you:

\documentclass{article}
\usepackage{booktabs,makecell,multirow,tabularx}
\renewcommand\theadfont{\bfseries\normalsize}
\newcolumntype{C}{>{\centering\arraybackslash}X}

\usepackage{geometry}

\begin{document}
    \begin{table}[ht]
\centering
\begin{tabularx}{\textwidth}{c L c L c L c }
    \toprule
\multirow{2}{*}{\thead{Atlas\\ Resolution}}
    &   \multicolumn{2}{c}{\thead{Variant-1}}
        &   \multicolumn{2}{c}{\thead{Variant-2}}
            &   \multicolumn{2}{c}{\thead{Variant-3}}    \\
    \cmidrule(lr){2-3}\cmidrule(lr){4-5}\cmidrule(lr){6-7}
    &   \thead[b]{Average\\ Render\\ Time (ms)}  &   \thead[b]{IFPS}
        &   \thead[b]{Average\\ Render\\ Time (ms)}  &   \thead[b]{IFPS}
            &   \thead[b]{Average\\ Render\\ Time (ms)}  &   \thead[b]{IFPS}                \\
    \midrule
512$\times$512      &  &  &  &  &  &  \\
1024$\times$1024    &  &  &  &  &  &  \\
    \addlinespace[2pt]
18432$\times$18432  &  &  &  &  &  &  \\
19456$\times$19456  &  &  &  &  &  &  \\
    \bottomrule
\end{tabularx}
    \caption{List of sizes for various resolution of Dataset-I, compressed variant Dataset-II and different slices variant Dataset-II}
    \label{tab:1}
\end{table}
\end{document}

Edit: I thoroughly redesign your table. In this I use packages booktabs for horizontal rules, makecell for column heads, tabularx for span table over text width and simple determination of columns width. The fonts in column headers is defined with

\renewcommand\theadfont{\bfseries\normalsize}

Comments of Mico remain me, that many people (among them me too) consider table more aesthetically if font in column headers are not in bold face. This is simple achieve with use:

\renewcommand\theadfont{\normalsize}

In this case the table look-out is: