Hello,
The set_paper takes two arguments : an array consisting of for float
numbers : x, y, width and height abd a second one, optional, with the
orientation of the paper.
I don't know what is the use for x and y, but width and height are
what you want.
They are expressed in Postscript points :
http://en.wikipedia.org/wiki/Point_%28typography%29
For example, 110mm in points is :
110 / 0.3527 = 311.879785pt
so you'll have :
$dompdf = new DOMPDF();
$dompdf->set_paper(array(0,0,311.88,311.88));
and you'll have a PDF of 110mm x 110mm