Define a new big operator in ConTeXt

I would like to replace the sum symbol by a big plus as Pedro G. Mattos wanted to do here How can I define a big plus operator that works like \bigcup?.

In the wiki I found that the command I should use is \definemathsymbol. However I am confused about how to adapt the symbol for both inline and display maths.

A solution might exactly be https://wiki.contextgarden.net/Product_integral but I do not get it and I do not want to use various commands such as prodi, PRODI... but only the sum one. Is it possible?

The code is very basic.

\starttext
Solve for \m{n} the following:
\startformula
\sum_{k=0}^n k = 6
\stopformula
{\bf Hint:} \m{\sum_{k=0}^2 k = 3}
\stoptext


If you solve the problem, an answer to replace prod by a big times would also be appreciated.

The is no such things as a “big plus“ in Unicode. However, there is a big times as per your second request.

\starttext
Solve for \m{n} the following:
\startformula
\bigtimes_{k=0}^n k = 6
\stopformula
\bold{Hint:} \m{\bigtimes_{k=0}^2 k = 3}
\stoptext


You can make your own big plus by just scaling the regular plus symbol. This is typographically not very consistent because it also scales the line width.

\define\bigplus{%
\mathop{%
\mathchoice
{\dobigplus{2000}}%
{\dobigplus{1500}}%
{\dobigplus{ 900}}%
{\dobigplus{ 700}}%
}%
}

\define[1]\dobigplus{%
\vcenter{\hbox{\scale[scale=#1]{$+$}}}%
}

\starttext
Solve for \m{n} the following:
\startformula
\bigplus_{k=0}^n k = 6
\stopformula
\bold{Hint:} \m{\bigplus_{k=0}^2 k = 3}
\stoptext