• General syntax for symbols with a kind of lower and upper limits:

\symbolname_{<em>lowerexpression</em>}^{<em>upperexpression</em>}

• In general, there are two ways how these lower and upper expressions can be placed: centered below and above the symbol or in a subscript / superscript manner. In the first case the symbol name is preceded by the word "big", in the second there is no prefix.
• Syntax for product symbol:

$$$\bigprod_{i=k}^{n}$$$   gives  

$$\bigprod_{i=k}^{n}$$

and

$$$\prod_{i=k}^{n}$$$   gives 

$$\prod_{i=k}^{n}$$

• Use font size commands for a nicer picture:

$$$\LARGE\bigprod_{\tiny{i=k}}^{\tiny{n}}$$$   gives  

$$\LARGE\bigprod_{\tiny{i=k}}^{\tiny{n}}$$ 

and

$$$\large\prod_{\small{i=k}}^{\small{n}}$$$   gives 

$$\large\prod_{\small{i=k}}^{\small{n}}$$