I try to answer your questions:
1) magnitude of a vector.
you have two coordinate systems , one inertial fixed and the other body fixed (airplane fixed).
let say you know the vector components ($\vec{v}_B$) in airplane system (index B) , so the magnitude of velocity vector is:
$$v_1=\sqrt{\vec{v}_B^T\,\vec{v}_B}\tag 1$$
we transfer this vector components to inertial system :
$$\vec{v}_I=R\,\vec{v}_B$$
where $R$ is the rotation matrix between body system and inertial system.
if we calculate the amplitude of this vector we get:
$$v_2=\sqrt{\left(R\,\vec{v}_B\right)^T\,R\,\vec{v}_B}=\sqrt{\vec{v}_B^T\,R^T\,R\,\vec{v}_B}=\sqrt{\vec{v}_B^T\,\vec{v}_B}\tag 2$$
with $R^T\,R=I_3\quad \Leftrightarrow v_2=v1$
thus the amplitude of a vector is invariant under orthonormale coordinate transformation (rotation matrix).
2) how to calculate the yaw pitch and roll angles
you can "build" the rotation matrix $R$ out of three elementar rotation matrices:
$$R_1(\alpha)=\left[ \begin {array}{ccc} 1&0&0\\ 0&\cos \left(
\alpha \right) &-\sin \left( \alpha \right) \\ 0&
\sin \left( \alpha \right) &\cos \left( \alpha \right) \end {array}
\right]
\quad \text{Rotation about the x-axes}$$
$$R_2(\beta)=\left[ \begin {array}{ccc} \cos \left( \beta \right) &0&\sin \left(
\beta \right) \\ 0&1&0\\ -\sin
\left( \beta \right) &0&\cos \left( \beta \right) \end {array}
\right]
\quad \text{Rotation about the y-axes}$$
$$R_3(\gamma)= \left[ \begin {array}{ccc} \cos \left( \gamma \right) &-\sin \left(
\gamma \right) &0\\\sin \left( \gamma \right) &\cos
\left( \gamma \right) &0\\ 0&0&1\end {array}
\right]
\quad \text{Rotation about the z-axes}$$
for example $R=R_3(\alpha)\,R_2(\beta)\,R_3(\gamma)$. notice that in this case the first rotation is about the z-axes, the second rotation is about the "new" $y'$-axes and the third is about is about the "new" $z''$-axes. these angles are call euler angles. for each rotation matrix you get singularity (gimbal) in one of the euler angles.
to obtain the yaw ($\psi$) pitch ($\vartheta$) and roll ($\varphi$) angels we choose this rotation matrix between the body system and inertial system
$$R_{BI}=R_3(\psi)\,R_2(\vartheta)\,R_1(\varphi)$$
the singularity of this matrix is wenn the pitch angle $\vartheta \pm \pi/2$