You could use the
relsize package and its
\larger command to scale up a bold-face question mark. In the following MWE, the default enlargement step is 1, but there's an option to change the amount of enlargement up to 5 (which corresponds to the
\Huge absolute-size setting command).
\bigqm ("big question mark") is set up using the
\text command because you have the term "mathematical symbols" in your question's title; the macro also works in text mode, by the way.
Question mark: ?