to adapt your example with the correct size bracket.
As for not being able to use TeX-like syntax to write equations on this site: this is a deliberate choice. Most of the time we actually want to talk about the TeX source here rather than the equations (or whatever) it produces; and there are times when it could be actively confusing to have a TeX-like process to produce images that isn't actually TeX (in its full, Turing-complete, confusing glory...)
One possibility: I used \DeclarePairedDelimiterX from the mathtools package to define a \MeijerM command with three arguments which is responsible to typeset the delimited matrix; then I defined \MeijerG having eight arguments (the first one is optional and will be passed as the optional argument to \MeijerM); using the \WithSuffix command from the suffix package to provide the starred version \MeijerG*:
The size of delimiters in the second and third examples is obviously wrong, but I just included them to test the functionality of the defined commands. Also, I used simple sub/superscripts to typeset the first four arguments, but of course you can use one of your proposed variants instead.