How to get a longer nmid

In the meantime I hope that you excuse me if I reply your question with these links, but I think that they help you to find half answer: Why does bigmid not work?, middle too big.

Here my MWE using scalerel package to have a big nmid. Into scaleobj{...} you can put a scale factor.

documentclass[a4paper,12pt]{article}
usepackage{amsmath,amssymb}
usepackage{scalerel}


begin{document}
[amathbin{scaleobj{1}{nmid}} b]
[begin{Vmatrix}
1 & 2 & 3
a & b & c
end{Vmatrix}mathbin{scaleobj{2}{nmid}} begin{Vmatrix}
7 & 2 & 1
a & b & c
end{Vmatrix}]

end{document}