How do I find the window dimensions and position accurately including decorations?

The following script will give you the top-left screen co-ords and size of the window (without any decoration). . . . xwininfo -id $(xdotool getactivewindow) contains enough information for you.

---

#!/bin/bash
# Get the coordinates of the active window's
#    top-left corner, and the window's size.
#    This excludes the window decoration.
  unset x y w h
  eval $(xwininfo -id $(xdotool getactivewindow) |
    sed -n -e "s/^ \+Absolute upper-left X: \+\([0-9]\+\).*/x=\1/p" \
           -e "s/^ \+Absolute upper-left Y: \+\([0-9]\+\).*/y=\1/p" \
           -e "s/^ \+Width: \+\([0-9]\+\).*/w=\1/p" \
           -e "s/^ \+Height: \+\([0-9]\+\).*/h=\1/p" )
  echo -n "$x $y $w $h"
#

Much simpler way to get current window size and position:

xdotool getwindowfocus getwindowgeometry

and to get selected window size and position:

xdotool selectwindow getwindowgeometry

The accepted answer can be extended to get the entire window:

entire=false
x=0
y=0
w=0
h=0
b=0  # b for border
t=0  # t for title (or top)

# ... find out what user wants then 

eval $(xwininfo -id $(xdotool getactivewindow) |
  sed -n -e "s/^ \+Absolute upper-left X: \+\([0-9]\+\).*/x=\1/p" \
         -e "s/^ \+Absolute upper-left Y: \+\([0-9]\+\).*/y=\1/p" \
         -e "s/^ \+Width: \+\([0-9]\+\).*/w=\1/p" \
         -e "s/^ \+Height: \+\([0-9]\+\).*/h=\1/p" \
         -e "s/^ \+Relative upper-left X: \+\([0-9]\+\).*/b=\1/p" \
         -e "s/^ \+Relative upper-left Y: \+\([0-9]\+\).*/t=\1/p" )
if [ "$entire" = true ]
then                     # if user wanted entire window, adjust x,y,w and h
    let x=$x-$b
    let y=$y-$t
    let w=$w+2*$b
    let h=$h+$t+$b
fi
echo "$w"x"$h" $x,$y

Although easy, it turns out not to work on Unity in Ubuntu 14.04 because the Relative info is all 0. I asked Get the full window dimensions (including decorations) in Unity and got a good answer. Here is how I used that answer:

aw=$(xdotool getactivewindow)
eval $(xwininfo -id "$aw" |
      sed -n -e "s/^ \+Absolute upper-left X: \+\([0-9]\+\).*/x=\1/p" \
             -e "s/^ \+Absolute upper-left Y: \+\([0-9]\+\).*/y=\1/p" \
             -e "s/^ \+Width: \+\([0-9]\+\).*/w=\1/p" \
             -e "s/^ \+Height: \+\([0-9]\+\).*/h=\1/p" )
if [ "$entire" = true ]
then
    extents=$(xprop _NET_FRAME_EXTENTS -id "$aw" | grep "NET_FRAME_EXTENTS" | cut -d '=' -f 2 | tr -d ' ')
    bl=$(echo $extents | cut -d ',' -f 1) # width of left border
    br=$(echo $extents | cut -d ',' -f 2) # width of right border
    t=$(echo $extents | cut -d ',' -f 3)  # height of title bar
    bb=$(echo $extents | cut -d ',' -f 4) # height of bottom border

    let x=$x-$bl
    let y=$y-$t
    let w=$w+$bl+$br
    let h=$h+$t+$bb
fi

This second method works in both Unity and Xfce, and should work in Gnome too.