Let $G:=GL_6(\mathbb{F}_q)$ where $F_q$ is a finite field of order $q=p^m$ for some prime $p$ and $m\in \mathbb{N}$. Consider ${}_2D_6(\mathbb{F}_q)$ as the collection of all matrices $A\in G$ where $A$ is of the form $$A=\begin{pmatrix} a_{11}&a_{12}&0&0&0&0\\ a_{21}&a_{22}&a_{23}&0&0&0\\ 0&a_{32 }&a_{33}&a_{34}&0&0\\ 0&0&a_{43}&a_{44}&a_{45}&0&\\ 0&0&0&a_{54}&a_{55}&a_{56}\\ 0&0&0&0&a_{65}&a_{66} \end{pmatrix}$$
Now how to find $|{}_2D_6(\mathbb{F}_q)|$?
I tried the following. Since all the diagonal entries must be non-zero so there are $(q-1)^6$ choices. But it is wrong since the matrix is not diagonal matrix, so this approach must fail.
How to find the order of the above set then ?
