I have to admit it's always been very clear to me, but I'm a weirdo about what stumps me and what doesn't.  Then again, I've completely ignored the idea that there's some objective scale to things, and that the internal Poser scale is wrong.  So I just go by what each piece of software says.  By default, 1 Blender unit = 1m.  When you import, select the object and it will say exactly how big it is in x, y, and z directions.  Poser has its own version of 1m or 1cm measurements (though I'll admit that at the time I did my estimates, I was working in inches).  My (very) rough estimate of scale between Poser and Blender is 2.5.  IIRC, accurate is smaller, with Poser's scale being closer to Blender's than that,  but I wanted an easy transform number (2.5 up into Blender, 0.4 out into Poser).  That said, while I scale up by hand when I import, I have settings on my export to scale down.  So I could probably work more accurately now, since I only need one sensible number (for instance 2.3 is simple, but 1/2.3 isn't).  That said, working at 2.5 scale has been OK for me, though a tad big with architecture sometimes.