When you buy a backdrop, it will probably last you years ... my advice is to get into 10x20 even if you are doing small groups ... there will be that one time you want to have a full body shot of the couple standing on the backdrop ... and a 10x10 will only work with the seven dwarfs. Now, if you are only doing small group shoulder shots or the seven dwarfs, ignore what I just said.
10x20 backdrop ... ze most popular size
With the 10x20 you can probably go both horizontal or vertical (depending on pattern) ... it's versatile and still light to carry.
Good for small groups vertically and larger gathering horizontally ("horizontally" I mean the backdrop, not the group).
For dancers etc. you might probably want to have a 20x20 so you have room to capture when they do their leaps and blah blah blah ... but still poses, you can use the 10x20 with the dancers standing on the cloth and look real purdy.
10x20 backdrop for families that get along
For my taste, the 10x20 is a bit small for an extended family ... 4-5 people yes, but once you start inviting grandpa, grandma, the aunt Sally from Wichita into the shot, they get pretty tight and need to get along real good.
If you work in a studio, maybe think about 20x20. I know setting up a 20x20 in a home setting can be a pain in the buttski.
20x20 and 20x30 backdrops for dance studios, team sports, corporate groups, etc.
When you need to photograph VLGs (very large groups), you need 20x20 and 20x30 ... hey, we've even done a 40x50 footer (took 2 guys to lift it).