Suppose we have two areas: $B$ of size $4m^2$ and $A$ of size $2m^2$. What is the ratio between their sizes?
A simple division would yield 2, but I think that the answer is 4, as illustrated below:

Could you please clear up the confusion? (E.g. in case the latter option is correct: If a simple division gives the wrong result, how should one approach this problem?)
The ratio between their areas is, as you said, 2. (The word "size" is a bit ambiguous, but it's most natural to interpret it as "area" in this context.)
If $A$ and $B$ are both squares, we could also ask the related (but different!) question: what is the ratio between their side lengths? In that case, $A$ would have side length $\sqrt{2} m$ and $B$ would have side length $2 m$, and the ratio of side lengths is $\frac{2}{\sqrt{2}} = \sqrt{2}$.