I'm Reform, and I generally wear a kippah when leading/participating in prayer or study, and when otherwise volunteering for my congregation. I know it's optional for women (of which I am one), but it has become a common minchag among non-Orthodox women at least in the past 25 years or so.
I recently learned to crochet kippot, using the construction method that involves spiraling outward from the middle until it "looks about right"--to my eye this is 6" diameter, more or less.
I asked a Conservative-Rabbi friend with whom I study Torah (who also crochets a lot of kippot) what the minimum size should be for a kippah. She said that it was a Very Good Question, but did not know. She too, works until it "looks right."
She's going to look into it, but I figure I'd ask here too: Halachically speaking, is there a minimum size for kippot? If so, what is it?