Not sure what state/country you are writing from but in the US, I would estimate the VAST majority of true dahlia lovers order from smaller individual growers, like Swan island, Aztec, Hollyhill, et al. Not only do you order only 1 tuber but their descriptions will tell you exactly how large the flowers are expected to be.

Assuming you are already familiar however with on line growers, I can only assume you deliberately choose the larger European growers like Bourgondien and Royal dutch (or perhaps you are in Europe and many of our growers will not ship out of country) for a reason. If you are purchasing pre-packaged tubers however, I can tell you that the amount of mismarking and misleading of packaged dahlias has been getting worse and worse. (see my recent post re: Rosella and Rosemunde. I just saw another Rosella at OSH today and once again, it is advertised as a dinnerplate and yet mine was at best 4-5" open.)