You can get all different kinds of sizes of parentheses, brackets, curly brackets, etc. to make your formulas look good. Typing
$$ \Biggl( \biggl( \Bigl( \bigl( ( ) \bigr) \Bigr) \biggr) \Biggr) $$gives you
Analogous commands work for [ and \{. Note
the 
used to get curly braces.
You can also let 
figure out how big a pair of
parentheses should be to fit the contained expression.
Use \left( and \right), like this:
$$ \left(\sum a_i^2\right)^{1/2} $$
gives
Of course, you can also use \left[, \right],
\left\{ and \right\}.