Terminator does allow you setting up a default size. :)
Just open the file ~/.config/terminator/config with your favorite editor.
You should have a section titled [layouts]. You only need to add a line within the sub-section named [[[window0]]] belonging to the [[default]] section. My configuration file, for instance, looks like this:
[global_config]
enabled_plugins = CustomCommandsMenu, InactivityWatch, ActivityWatch, TerminalShot, LaunchpadCodeURLHandler, LaunchpadBugURLHandler
[keybindings]
[profiles]
[[default]]
scrollbar_position = hidden
visible_bell = True
scrollback_lines = 1000000
foreground_color = "#ffffff"
copy_on_selection = True
background_color = "#300a24"
[layouts]
[[default]]
[[[child1]]]
type = Terminal
parent = window0
profile = default
[[[window0]]]
type = Window
parent = ""
profile = None
size = 900, 600
[plugins]
The relevant part is only the line starting with "size", of course.