This snippet uses i3ipc python module to print dimensions of just focused container

main.py

#!/usr/bin/env python3

from i3ipc import Connection, Event

i3 = Connection()


def print_dimensions(i3, e):
    try:
        con = i3.get_tree().find_focused()
        rect = con.rect
        print("width: {}, height: {}".format(rect.width, rect.height))

    except Exception as e:
        print('Error: {}'.format(e))


def main():
    i3.on(Event.WINDOW_FOCUS, print_dimensions)
    i3.main()


if __name__ == "__main__":
    main()