The spec states:

A named constructor MUST have a property named “length” with attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false } whose value is a Number determined as follows:

1. Initialize S to the effective overload set for constructors with identifier id on interface I and with argument count 0.
2. Return the length of the *shortest* argument list of the entries in S.

http://dev.w3.org/2006/webapi/WebIDL/#named-constructors