Poster board won't work--it's too flimsy.
I use 1/4 inch foam core board--you can get this at any craft or art supplies store. Office supply stores will have them too. 4 feet diag. should be large enough, rectangular in shape is better than square (helps clarify orientation to Mocap Studio.) Finish should be matte, not glossy because gloss will reflect IR. Color doesn't matter unless it's black because black will 'absorb' light. I use white but if you can get a light to med gray, that's probably better. White color is fine though, so long as lighting isn't too bright in the room. (I usually record mocap at night with normal 'dim' room lighting.)
If you can only get high gloss, a good example for finish is what I did with our head tracking helmet as shown on our blog (
http://littlegreendog.blogspot.com/). The original finish of the helmet was glossy white which is completely unsuitable for Kinect. Painting it flat gray as shown in the photo works much better.
Most likely, the finish on the foam core you buy will be fine though--they typically come with a matte finish.
G.
