Thank you! I was looking for this.
Then , , can be written via
$\newcommand{\bigplus}{\mathop{\Large+\normalsize}} \bigplus_{i \in I}Vi$
$\newcommand{\bigdot}{\mathop{\Large•\normalsize}} \bigdot_{i \in I}Vi$
$\newcommand{\bigmult}{\mathop{\Large*\normalsize}} \bigmult_{i \in I}Vi$
This short post is about how to write something like and
MathJax doesn't allow picture environments so we can't define a new command that draws a cross. The solution is simple: Enlarge the normal \times command.
Paste this in a math environment at the top of the document:
\newcommand{\bigtimes}{\mathop{\Large\times\normalsize}}