It is really a measure of a solution to a problem.
Functional measures try to measure the size of the problem itself.
Function point analysis is widely used, but only applies to some types of software. Research here is investigating functional size measures. One project investigates the internal structure of FPA, to see how the method might be made more effective or simpler. Another project proposes a functional size measure for object oriented software. Other research looks at the use of software size estimates, for estimating development effort and cost.