You can pass an init object. Remeber that _x and _y positions are relative to the MovieClip you pas as parent:
The command is:
PopUpManager.createPopUp(parent, class, modal [, initobj, outsideEvents])
An example:
win = mx.managers.PopUpManager.createPopUp(this, mx.containers.Window, true,{_name: "myWin", _x:5, _y:125, _width:300, _height:250, closeButton:true, title:"Details", contentPath:"details"});
Jorge