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

\symbolname_{lowerexpression}^{upperexpression}

• 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 summation symbol:

$$\bigsum_{i=k}^{n}$$   gives  

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

and

$$\sum_{i=k}^{n}$$   gives 

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

• Use font size commands for a nicer picture:

$$\LARGE\bigsum_{\small{i=1}}^{\small{n}}$$   gives  

$\LARGE\bigsum_{\small{i=1}}^{\small{n}}$

and

$$\large\sum_{\small{i=1}}^{\small{n}}$$   gives 

$\large\sum_{\small{i=1}}^{\small{n}}$