Finder won't remember window size in Yosemite
I don't know any way to change the default window size, but you can use a script like this to reset the properties of windows manually:

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, 22, 1920, 1076}
        tell its column view options
            set shows icon to false
        end tell
    end tell
end tell