The job of a radar reflector is to reflect. How well it does this depends on the size of the reflector and the wavelength of the radar signal. If the reflector is smaller than the wavelength of the signal, it's not going to do much. With radar reflectors, bigger is better.
Keep in mind that different parts of the marine community use different radars. The coastal community (recreational) uses X band radar, with a wavelength of about 1"-1.5". The offshore guys (commercial) use S band stuff, with a wavelength of 3"-6". Although I've seen submarines coming into New London that have had an apparently supplemental X band radar rigged.
So, who do you want to see you? If just X band, a 2-3" reflector diameter will be effective - but you're not going to reflect the commercial S band stuff very well. If you're going to show up on S band radar, you need something like 9-12".
Personally, I use a 12" Davis reflector. I'm careful to hang it in the 'catch rain' position so a signal gets to see 3 faces. I haven't gone so far as to hang two of these oriented 45 deg from each other, although that would be an improvement. Reflectors seem to vary their effectiveness depending on the incident angles & using two at different angles would be better than one.
This is not just my personal opinion, the supporting data is out there.