There is a standard way to size the functional components of software: Function Point Analysis. Function points, by their very name, focus on the delivered functionality of software - thus, they do not account for non-functional development.
However, most Certified Function Point Specialist have encountered projects with significant development requirements for non-functional work.
Thus, IFPUG developed SNAP, the Software Non-functional Assessment Process, which supplements the Funtion Point Analysis sizing methodology and provides a sizing technique for the non-functional component of software.
This month's Trusted Advisor report outlines the SNAP framework and discusses how it can be used for sizing non-functional requirements - and why it's best used in tandem with Function Point Analysis.