You will always use Pythagoras theorem to find magnitude while given components of vector.
No matter if system that you are observing is 2D or 3D or higher (which is imaginery) pythagoras theorem will be applied.
Formula for determining magnitude is:
[tex]M = \sqrt{x^ + y^2 + z^2 + ...} [/tex]
Number of turms inside square root you take depending on dimension of your system.
