The principal variable for customizing indentation is the
style variable c-offsets-alist, which gives an
offset (an indentation rule) for each syntactic
symbol. Its structure and semantics are completely described in
c-offsets-alist.
The various ways you can set the variable, including the use of
the CC Mode style system, are described in Config Basics and its
sections, in particular Style Variables.
The simplest and most used kind of “offset”
setting in c-offsets-alist is in terms of multiples
of c-basic-offset:
This style variable holds the basic offset between indentation levels. It's factory default is 4, but all the built-in styles set it themselves, to some value between 2 (for
gnustyle) and 8 (forbsd,linux, andpythonstyles).
The most flexible “offset” setting you can make in
c-offsets-alist is a line-up function (or even a
list of them), either one supplied by CC Mode (see Line-Up
Functions) or one you write yourself (see Custom
Line-Up).
Finally, in Other Indentation you'll find the tool of last resort: a hook which is called after a line has been indented. You can install functions here to make ad-hoc adjustments to any line's indentation.