A sheeet could work for smaller subjects or head and shoulder shots, when you get into full body with teens or adults, you should have something larger
I use two white king sized sheets and also two black king sized sheets when I want those colors. I also have 4 muslin 10x20 backdrops I ordered from ebay. Eventually I would like to get real black and white muslin - but for now the sheets are working fine.
I just got 2 black sheets...wanted to see how they would work. Right now, it will be mainly pictures of my kids... but i do want to get some muslin backdrops.