math-size displaystyle

[Eric Delevaux]

Hi list
Back with another problem :)
I can't use the math-size displaystyle like in man, 5.6.3.
It does work in the man, but i type "math-size displaystyle" in my equation and 
nothing happens. I saw previous messages in archive but no solutions (except 
redefining something in preamble and then ert...)
Does anyone use this?
Thanks Eric