I would like to make the vertical bars in the \draw[|<->|]
command a bit longer. Can I do that with little editing to the following code?
\documentclass{amsart}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{calc,intersections}
\usepackage{array, boldline, makecell}
\setlength{\oddsidemargin}{0.0in}
\setlength{\evensidemargin}{0.0in} \setlength{\textwidth}{6.1in}
\setlength{\topmargin}{0.0in} \setlength{\textheight}{9in}
\begin{document}
\noindent \hspace*{\fill}
\begin{tikzpicture}
%A rectangle of length b + c and width a is drawn, and below it, two rectangles of lengths b and c and width a are drawn.
%(The figure is magnified by a scale of 2/3.)
\path (0,0) coordinate (A) ({(2/3)*15},0) coordinate (B) ({(2/3)*15},{(2/3)*3}) coordinate (C) (0,{(2/3)*3}) coordinate (D);
\path[fill=Dandelion] (A) -- ($(A)!2/5!(B)$) -- ($(C)!3/5!(D)$) -- (D) -- cycle;
\path[fill=Turquoise] ($(A)!2/5!(B)$) -- (B) -- (C) -- ($(C)!3/5!(D)$) -- cycle;
\draw[dashed] ($(A)!2/5!(B)$) -- ($(D)!2/5!(C)$);
\draw (A) -- (B) -- (C) -- (D) -- cycle;
\draw[|<->|] ($(A)!4mm!90:(D)$)--node[fill=white,sloped] {$a$} ($(D)!4mm!-90:(A)$);
\draw[|<->|] ($(D)!4mm!90:($(D)!2/5!(C)$)$)--node[fill=white,sloped] {$b$} ($($(D)!2/5!(C)$)!4mm!-90:(D)$);
\draw[|<->|] ($($(D)!2/5!(C)$)!4mm!90:(C)$)--node[fill=white,sloped] {$c$} ($(C)!4mm!-90:($(D)!2/5!(C)$)$);
\draw[|<->|] ($(A)!4mm!-90:(B)$)--node[fill=white,sloped] {$b+c$} ($(B)!4mm!90:(A)$);
\node at ($($(A)!1/5!(B)$) +($(A)!1/2!(D)$)$){$ab$};
\node at ($($(A)!7/10!(B)$) +($(A)!1/2!(D)$)$){$ac$};
\end{tikzpicture}
\end{document}
[->{|[width=2cm]}|]
might work