how to get lenght of two floats
FVector2D(X,Y).Size() should do the job
Function you trying to use is for blueprint as FVector2D is outside of reflection system so it needs to have blueprint funciton nodes outside of it’s class. In C++ you should use functions declered direcly to struct or class, only primitives like float or int needs external functions (which are concentrated in FMath btw), look on API refrence
Also as you can see you can construct variables on the go with constructor like FVector(X,Y,Z) or FVector2D(X,Y) or FRotator(Pitch, Yaw, Roll) so there no need for some special function overloads, you rarely gonna see them.
thank you