One option would be to scale it (and change the shorten
value accordingly):
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows, decorations.markings}
\tikzset{
vecArrow/.style={
thick,
decoration={markings,mark=at position
1 with {\arrow[scale=2,thin]{open triangle 60}}},
double distance=1.4pt, shorten >= 10.5pt,
preaction = {decorate},
postaction = {draw,line width=1.4pt, white,shorten >= 4.5pt}
},
innerWhite/.style={
semithick,
white,
line width=1.4pt,
shorten >= 4.5pt
}
}
\begin{document}
\begin{tikzpicture}[thick]
\node[draw,rectangle] (a) {A};
\node[inner sep=0,minimum size=0,right of=a] (k) {}; % invisible node
\node[draw,rectangle,right of=k] (b) {B};
\node[draw,rectangle,below of=a] (c) {C};
% 1st pass: draw arrows
\draw[vecArrow] (a) to (b);
\draw[vecArrow] (k) |- (c);
% 2nd pass: copy all from 1st pass, and replace vecArrow with innerWhite
\draw[innerWhite] (a) to (b);
\draw[innerWhite] (k) |- (c);
% Note: If you have no branches, the 2nd pass is not needed
\end{tikzpicture}
\end{document}

I changed the old \tikzstyle
to the more appropriate \tikzset
syntax.