If you're buying a real steel Magpul CTR, you need the Civilian one. I think it's the larger of the two and closest in size to most paintball stocks.

Most airsoft Magpul CTR stocks (The actual Magpul PTS stock and subsequent replicas) are civilian size. You can get replicas cheaper than the real steel ones if you're interested.

Also, the Rap 4 A5 stock buffer tube adapter will work with the Magpul CTR no problem. This will be cheaper than buying a new A5 M5 collapsible stock, using the buffer tube AND buying a CTR.