Here it is

wmctrl -i -r $(xwininfo | egrep -om 1 '0x[0-9a-z]+') -e 0,$X,$Y,$WIDTH,$HEIGHT

You can make alias for this one-liner or use this win-resize.sh script:

#!/bin/bash
# Using xwininfo and wmctrl to resize a window
# Written by Yu-Jie Lin
# Public Domain

WIN_ID=$(xwininfo | egrep -om 1 '0x[0-9a-z]+')
W=$1
H=$2
X=${3:--1}
Y=${4:--1}

wmctrl -i -r $WIN_ID -e 0,$X,$Y,$W,$H

xwininfo is used for the Window ID only, its click-on-target method fits into the targeted window acquirement very well. Since the first hex string is the Window ID in xwininfo output, therefore the parsing stops when first match occurs (-m 1). -o is to have only matched string printed from egrep. The script accepts two parameters, width and height; position X and Y are optional, will be set to -1 if missing.

I wrote this script for screenshot of window, I want to take shots with window in specific size. There might be some issues with your window manager and screen grabber. I use dwm and xsnap, there is no window decoration to be grabbed since dwm does not have such thing except a border. You will need to adjust some settings in order to keep final image in desired width and height.