function mmToPixels(mm:Number):uint
{
   return Math.round(Capabilities.screenDPI * (mm / 25.4));
}


var myWidthInMillimeters:Number = 38;
var myWidthInPixels:uint = mmToPixels(myWidthInMillimeters);
trace("myWidthInPixels: "+myWidthInPixels);

// OUTPUT
// myWidthInPixels: 108

http://www.adobe.com/devnet/flash/articles/authoring_for_multiple_screen_sizes.html

This is taken from Christian Cantrell's Adobe article called 'Authoring mobile Flash content for multiple screen sizes'.

AS3 Millimeters to Pixels

ActionScript 3