By definition of magnitude of cross product:
$\| \mathbf{F} \times \mathbf{r} \|= \| \mathbf{F} \|\ \| \mathbf{r} \| \sin (\mathbf{F},\mathbf{r}) \tag1$
$\| \mathbf{r} \times \mathbf{F} \|= \| \mathbf{r} \|\ \| \mathbf{F} \| \sin (\mathbf{r},\mathbf{F})\tag2$
By $(1)$ and $(2)$: $\| \mathbf{F} \times \mathbf{r} \|=-\| \mathbf{r} \times \mathbf{F} \|\tag3$
$\| \mathbf{F} \times \mathbf{r} \|$ and $\| \mathbf{r} \times \mathbf{F} \|$ are modes of vectors.
$\| \mathbf{F} \times \mathbf{r} \|$ and $\| \mathbf{r} \times \mathbf{F} \|$ are positive.
Then how can equation $3$ be true?