A good place to start is the LimitRequestBody directive. This directive is set to unlimited by default. If you are allowing file uploads of no larger than 1MB, you could set this setting to something like:
If you're not allowing file uploads you can set it even smaller.
Some other directives to look at are LimitRequestFields, LimitRequestFieldSize and LimitRequestLine. These directives are set to a reasonable defaults for most servers, but you may want to tweak them to best fit your needs. See the documentation for more info.