Applescript to resize all windows from a given application. Source credit given in the top.

resize_window.scpt

(*
 
From http://www.labnol.org/software/resize-mac-windows-to-specific-size/28345/
This Apple script will resize any program window to an exact size and the
window is then moved to the center of your screen.  Specify the program name,
height and width below and run the script.
 
Written by Amit Agarwal on December 10, 2013
 
*)

set theApp to "MacVim"
set appHeight to 800
set appWidth to 1400

tell application "Finder"
	set screenResolution to bounds of window of desktop
end tell

set screenWidth to item 3 of screenResolution
set screenHeight to item 4 of screenResolution

tell application theApp
	activate
	reopen
	repeat with x from 1 to (count windows)
		set xAxis to 20 * x as integer
		set yAxis to (20 * x) as integer
		set the bounds of the window x to {xAxis, yAxis, appWidth + xAxis, appHeight + yAxis}
	end repeat
end tell