Regarding the age of the leaves, they are basically maximum size by the time the flowers open. The ones in the picture were not going to get any larger. I like to time my propagations for when the flowers are young (about to open or just having opened) as the late-stage inflorescences give inferior results. The other thing related to timing is that you want to try to get the leaves started when they are as plump as possible (ideally 2-3 days after watering the plant). In the case of the one pictured in the OP, now is the time, not later. You do not necessarily need to do surgery, sometimes the leaves come right off the stalk by pulling. The key is to get the base of the leaf intact because that's where the new plantlets come from.
