seed gathering?

[Oscgrr]

I tried to gather seeds from a supermarket tomato (just to see if it could be done) this past Spring and they never sprouted. Over the Summer, I researched and learned that the seeds needed to be fermented. Okay, that explained that but I read on and the article recommended that any seeds that I wish to save should be taken from one of the best tomatoes on the plant. At the start of the season, I bought several plants from a big hardware/lumber/garden store and on a whim I picked up a yellow tomato plant that claimed to be an heirloom (I didn't know what "heirloom" was, I do now) and it gave me a bunch of very nice deep yellow, fist sized tomatoes that tasted awesome. I ate just about all of them before I decided that I would like to start all my plants from seed next year but the only one left was a smallish tomato with several surface cracked looking scars. I put the seeds in a jar with some flesh and a little water and covered it with cheesecloth. It looks fermented and smells nasty, like I believe it's supposed too. My question is this, because I saved the seeds from a smaller fruit, will that mean that my tomatoes next year may trend smaller? I understand that there's no guarantee and genetics is a tricky thing to predict but more specifically, do tomatoes really differ, genetically, from fruit to fruit on the same plant? If I wanted larger fruit, should I have taken seeds from the largest one on the plant?