Tikzpicture - how to make major ticks longer or thicker

Question

I am trying to make the major ticks (11,12,13,14,15,16,17,18,19 and 20) either thicker or longer so that the box plot statistics are easier to see. Does Anyone know how to achieve this?

\documentclass{article}
\usepackage[margin=0.5in]{geometry}
\usepackage{pgfplots}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage{tikz}
\usepackage{amssymb}
\usetikzlibrary{patterns}
\usepackage{xcolor}
\usetikzlibrary{arrows}
\usepgfplotslibrary{fillbetween}
\usepgfplotslibrary{statistics}
\usetikzlibrary{shapes.misc}
\begin{document}
\begin{tikzpicture}
\pgfmathsetlengthmacro\MajorTickLength{
    \pgfkeysvalueof{/pgfplots/major tick length} * 4
}
\begin{axis}
[
ytick=\empty,
xmin=11,
xmax=20,
xmajorticks=true,
minor x tick num=4,
xtick={11,12,13,14,15,16,17,18,19,20},
axis x line=bottom,
axis line style={latex-latex},
axis y line=none,
enlargelimits=0.05,
height=3.5cm,
width=13cm,
clip=false
]

\addplot[
yshift=0.1cm,
boxplot prepared={
median=15.5,
upper quartile=17.6,
lower whisker=11,
lower quartile=13.6,
upper whisker=19,
whisker extend=0 % height of whiskers
},black
] coordinates {};
%
\end{axis}
\end{tikzpicture}
\end{document}

Thank you!

Answer 1

On page 342 from the manual (version 1.16), it introduces every minor tick/.append style={...} and every major tick/.append style={...} options for customization. You need to use minor tick length = something and major tick length= something inside. I have condensed your code into a minimal example.

PS You don't need \pgfmathsetlengthmacro\MajorTickLength{...}.

\documentclass{article}
\usepackage[margin=0.5in]{geometry}
\usepackage{pgfplots}
\usepackage{tikz}
\pgfplotsset{compat = newest} % Current version is 1.16

\usepgfplotslibrary{statistics}
\begin{document}
\begin{tikzpicture}
\begin{axis}
[
ytick=\empty,
xmin=11,
xmax=20,
minor x tick num=4,
xtick={11,12,13,14,15,16,17,18,19,20},
%%%%%%%%%%%%%%%%%%%% What you need:
every major tick/.append style={very thick, major tick length=10pt, black},
every minor tick/.append style={thick, minor tick length=3pt, red},
%%%%%%%%%%%%%%%%%%%%
axis x line=bottom,
axis line style={latex-latex},
axis y line=none,
enlargelimits=0.05,
height=3.5cm,
width=13cm,
clip=false,
]
\addplot[
yshift=0.2cm,
boxplot prepared={
median=15.5,
upper quartile=17.6,
lower whisker=11,
lower quartile=13.6,
upper whisker=19,
whisker extend=0 % height of whiskers
},black
] coordinates {};
%
\end{axis}
\end{tikzpicture}
\end{document}

Answer 2

With \pgfmathsetlengthmacro\MajorTickLength{...} you just created a length and stored some value to it, but you never applied that value somewhere. So by adding major tick length=\MajorTickLength to the axis you will get your disired result.

Please note that I heavily simplified your code -- as M. Al Jumaily did in his answer as well -- to achieve the same result ...

% used PGFPlots v1.16
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
    \usepgfplotslibrary{statistics}
\begin{document}
\begin{tikzpicture}
    \pgfmathsetlengthmacro\MajorTickLength{
        \pgfkeysvalueof{/pgfplots/major tick length} * 4
    }
    \begin{axis}[
        height=4cm,             % <-- (adjusted)
        width=13cm,
        axis x line=bottom,
        axis y line=none,
        axis line style={latex-latex},
        xmin=11,
        xmax=20,
        xtick distance=1,       % <-- (added)
        minor x tick num=4,
        ytick=\empty,
        major tick length=\MajorTickLength, % <-- added
        enlarge x limits=0.05,  % <-- (changed)
        enlarge y limits=0.25,  % <-- (added and adjusted)
    ]

        \addplot[
            boxplot prepared={
                median=15.5,
                upper quartile=17.6,
                lower whisker=11,
                lower quartile=13.6,
                upper whisker=19,
                whisker extend=0, % height of whiskers
            },
            black,
        ] coordinates {};

    \end{axis}
\end{tikzpicture}
\end{document}