
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows, chains, calc, positioning, shapes.multipart}
\begin{document}
\begin{tikzpicture}[
node distance = 9mm and 12mm, % <vertical distance> and <horizontal distance> between nodes
start chain = going right,
mw/.style = {minimum width=#1},% for shortcut in "list" shapes
list/.style = {rectangle split, rectangle split parts=2,
rectangle split horizontal, draw,
align=center,
text width=7mm, % <-- have effect only on the foirst part of shape
minimum height=9mm, % define height od multi part shape
inner sep=1mm, on chain},
> = stealth,
]
\node[list] (A) {1\nodepart[mw=7mm]{two} };
\node[list] (B) {2\nodepart[mw=7mm]{two} };
\node[list] (C) {2\nodepart[mw=7mm]{two} 3};
\node[list, below=of A] (D) {4\nodepart[mw=7mm]{two} };% <-- observe correct syntax of "positioning" library
\node[list] (E) {5\nodepart[mw=7mm]{two} };
\node[list] (F) {6\nodepart[mw=7mm]{two} 3};
\draw[*->] ($(A.two north)!0.5!(A.two south)$) -- (B);
\draw[*->] ($(B.two north)!0.5!(B.two south)$) -- (C);
\end{tikzpicture}
\end{document}
of course, it is simpler just increase inner sep
as suggest AboAmmar in his answer, but with his solution can width of shape parts wary. Anyway, see how the code of arrows are changed. Your approach is at least strange :).
Edit:
I expand above MWE with case of vertical for vertical align. Distance between nodes are determined with node distance
.
I also change the numbers in shape part, that you can see, that the width of parts is independent of it content (when it is shorter then `text width, of course).
rectangle split horizontal
you can only determine height of shape for whole shape. width can be determined only for each part separately as for example\nodepart[minimum width=7mm]{second}
minimum height=...
, you should look in PGF & TikZ manual. I will prepare short answer.