setting window geometry with xprop

I'm trying to set a window's geometry (position+dsize).
There's wmctrl & xdotool but their geometry isn't always accurate, so trying xprop.

Code:xprop -name firefoxshows _NET_WM_USER_TIME_WINDOW 0x0480b6c8

Trying to set its geometry with
Code:xprop -id 0x0480b6c8 -format _NET_WM_OPAQUE_REGION 16i -set _NET_WM_OPAQUE_REGION 100,100,500,500but nothing happens.

any ideas?

Thanks