[XeTeX] book size options --- help

[Nicolas Vaughan]

Dear all,
This a problem I've had ever since I began working in LaTeX, and I
still have it in XeTeX. Perhaps someone can help me with it.
I have been requested to typeset in XeLaTeX a math book of this size:
170mm x 240mm. I must have these margins: top: 25mm, bottom: 30mm;
outside: 25mm; inside: 25mm.
But I don't know exactly how to change this parameters.
Should I use the geometry package? Or the memoir class?
Any recommendations?
Thanks in advance.
Cheers!
Nicolas Vaughan