After a little experimentation I have a very simple answer.
in Report Properties...
set Auto Center to Yes.
set Auto Resize to No.
set Pop Up to Yes.
and in the line immediately following your DoCmd.OpenReport statement put...
DoCmd.MoveSize , , width, height (I use 13300 and 11000 on an XGA monitor)
If you want to specify the exact start position on the screen use
DoCmd.MoveSize left, top, width, height (0,0 is the left,top of screen)
if you want the start position to be relative to the active window and not the screen then set Pop Up to No.
Regards Brett