Two basic options are:
- Add an appropriate
row sep
.
- Add a distance after
\\
, e.g. \\[1.5cm]
.
These options do not do precisely the same in the sense that the distance specified has different meanings as is illustrated in the following example.
\documentclass{article}
\usepackage{tikz-cd}
\begin{document}
\[\begin{tikzcd}[column sep=50pt,row sep=50pt]
A \arrow{d}\arrow{r} & B \arrow[leftarrow]{dl}\\
C
\end{tikzcd}
\qquad\begin{tikzcd}[column sep=50pt]
A \arrow{d}\arrow{r} & B \arrow[leftarrow]{dl}\\[50pt]
C
\end{tikzcd}
\]
\end{document}

The first option might be better to set a universal row separation, the second one if you only want to change one out of several.
row sep
? Or replace ` \\` by ` \\[2cm]`.