I would like to add my 0.02 Euros (almost 0.03$ :lol:) to the discussion. As I am toying with the idea of creating props for Poser I searched for references and I found three relevant sources:
1 - Morphography tutorial page (with an extensive database of models heights in OBJ units);
2 - the 1988 U.S. Army Anthropometry survey (ANSUR) Final report which you can download from the URL given at this page;
3 - bagginsbill posts about shaders scales.

In the ANSUR publication (the relevant part starts at page 270, page 283 of the PDF), we see that the 50th-percentile of the male heights is 176 cm and the 99th-percentile is 191 cm. Considering that the report dates back to 1988, an average US anglosaxon male height of ~180 cm is reasonable taking into account the average height increase with time and above-the-average fitness for the Poser figures; the 180 cm measure fits also well that vague nearly-6-feet (183 cm) size reference in the Poser 4 manual.

P4 Male is 0.752517 PNU tall and this gives us ~239 cm/PNU (~94 inches/PNU) which is very near the well-known 96 inch/PNU scale; if we try the 103.2 inch/PNU scale P4 height becomes 77.7" (197 cm) which is outside any average unless you consider basket players.

The source of the problem is clear when you notice that the P4 male is one of the tallest meshes availble for Poser (check the table in the Morphography reference); as reported in that page, if we posit that the "average male" the manual writes about is not P4 male but a mythical figure 0.6975 PNU tall (never actually released), everything falls nicely in place with the 103.2 inch scale.

So now we can...
1 - ...model props using the "right scale" (103.2 in/PNU), having then to deal with a population of giants or...
2 - ...we can assume other scales in order to make props look right when compared to figures (and in this case probably the 96 inch/PNU is the most sensible one) or...
3 - ...considering that we are not creating items for actual fabrication and use by real human beings but just going for the looks right effect, select the figure you want to use as reference, pose it appropriately and export the resulting OBJ to use as reference size in your modeling application.

I think that the third option is the simplest (and least contentious) one.

P.S.: if we want people to use actual dimensions in Poser UI (like what is done in P.I.C.K.), we have no alternative but to go for the 103.2 inch/PNU scale which appears to be hardwired in Poser UI.

GIMP 2.7.4, Inkscape 0.48, Genetica 3.6 Basic, FilterForge 3 Professional, Blender 2.61, SketchUp 8, PoserPro 2012, Vue 10 Infinite, World Machine 2.3, GeoControl 2