Rearrange the parts of the sentence in correct order.
"The reference"
P - is simultaneously a matter
Q - to the size of a vulnerable community
R - of legal as well as political and ethical perspective

Let's trace the syntactic connections from the opening phrase:
"The reference..." is naturally followed by the preposition "to" to specify what is being referred to. Therefore, Q ("to the size of a vulnerable community") comes first.

After defining the subject ("The reference to the size of a vulnerable community"), we need the main verb. Part P provides this with "is simultaneously a matter".

"a matter..." needs the preposition "of" to explain what kind of matter it is, which is provided by part R ("of legal as well as political and ethical perspective").

The complete sentence is: "The reference to the size of a vulnerable community is simultaneously a matter of legal as well as political and ethical perspective."
Hence, the correct sequence is QPR.