I saw a tiny joule thief here: http://www.bigclive.com/joule.htm
. I wanted to replicate the size, but leave the application open for use in different projects. By following this excellent instructable: https://www.instructables.com/id/Make-a-Joule-Thief/
I was able to fit my circuit inside a 5x5 square of perfboard. It happily powers 3 leds in parallel.