Does the required number of reviewers depend on the size of the code being reviewed?
Not really. If your code is particularly important, then you might consider inviting a few more reviewers, or you might make a particular effort to select reviewers with the most experience.
However, if the code is too large, you don't need more reviewers—you need a rewrite!
Which tools are useful aids for code reviewing?
Common sense, a keen pair of eyes, and an alert brain!
A number of software tools are also useful. Many different tools can inspect your code and help you to gauge its quality and relative risk to the entire codebase. They can trace the flow of execution, work out which code is executed most often, and calculate ...