Calculating Amplitude of Elliptically Polarized EM Wave

[TriTertButoxy]

Suppose I have an eliptically polarizied EM wave, whose electric field can be written as

$$\vec{E}=A\cos(kz-\omega t)\vec{e}_x+B\sin(kz-\omega t)\vec{e}_y,$$

where the constants $A$ and $B$ are amplitudes of the $x$ and $y$ components of the beam, respectively.
How would I report the amplitude of the whole beam?

(Please note that I am not looking for intensity of the beam -- I know how to report that).

 

[mezarashi]

This question is similar to asking, what is the magnitude of a voltage source with time function:

$$V = A\cos (\omega t) + B\sin(\omega t)$$

There are two out of phase components superimposed on each other. In my opinion, it would only make sense to have some RMS value if you are doing power calculations.