They can definitely reach sizes up to a foot, plus or minus a couple inches. At where I work we keep a large school, of about 12 or 13 tin-foil barbs, and a few of them are at least 8 inches. They are such timid, fast swimmers it's tough to get a measurement. The ones at my work are not fully grown, they have been in the tank for maybe 3 years, and were bought when they were a year or 2 old. They live, I believe, if kept well up to 10 years.
It is hard to tell you "exact" maximum size...because all fish grow to different lengths depending on how they are kept and their health.
Hope that helped.