If you are looking strictly for the largest flowered petunias from seed, you should probably try to find the Limbo Series - they typically have a 4-5" bloom. I'm personally not a big fan of grandiflora petunias, mostly because the large blooms really take a beating in the rain, the petals are thinner, and they don't self clean, which leaves them open to get botrytis. Hem Genetics is the actual breeder of the Limbo Series - I'm not sure who carries them in the US for retail sales.

Here is a link that might be useful: Hem

I'll check them out. I mostly want them for hanging baskets that will be somewhat protected. I agree about the untidyness of them tho.