MacOS – Is it possible to keep a MacOS application window “Always in the Back”

This doesn't work with Preview or other aplications that don't support AppleScript.

tell application (path to frontmost application as text)
    try
        set miniaturized of windows where visible is true and index is not 1 and miniaturizable is true to true
    end try
    try -- at least Finder
        set collapsed of windows where visible is true and index is not 1 and closeable is true to true
    end try
end tell

I don't know how to get the focused window or hidden windows with System Events. The focused window isn't window 1 or AXMain or AXFocused. Setting AXMinimized to true didn't work with Tweetbot either.

tell application "System Events" to tell (process 1 where frontmost is true)
    windows where value of attribute "AXMinimized" is false
    --repeat with w in result
    --try
    --set value of attribute "AXMinimized" of w to true
    --end try
    --end repeat
end tell

There are many ways to assign shortcuts to scripts, but shortcuts for Automator services don't work reliably in 10.7 or 10.8.

In Terminal you can change the default window size from Preferences > Settings > Window:

You can also use window groups to set a default position and size for the window opened when you launch Terminal (but not for new windows opened later).

I use Sequential and Skim as alternatives to Preview and iTerm 2 as an alternative to Terminal. All of them have an option to open windows in full screen by default, and all of them have their own (pre-Lion and actually usable) implementations for full screen windows.

The sizes of Finder windows are saved in .DS_Store files and there are no default window size settings in com.apple.Finder.plist or anything. I use a script like this to reset the properties of windows:

tell application "Finder"
    if number of Finder windows is 0 then return
    tell Finder window 1
        set toolbar visible to false
        set sidebar width to 0
        set statusbar visible to false
        set current view to column view
        set bounds to {960, 44, 1920, 1076}
        tell its column view options
            set shows icon to false
        end tell
    end tell
end tell