How to prove a pseudo-random function has certain expansion factor
The concatenation operator $||$ in e definition $F$ ensures an expansion $n \rightarrow 2n$ since both $F_0$ and $F_1$ map $n$ bits to $n$ bits.

The argument for $F'$ is similar since repeated application of two $n$ bit to $n$ bit functions gives an $n$ bit to $n$ bit function, and four of those are concatenated in $F'$.