Indeed, one of the questions in the data entry portal will ask you for the diameter of the jar.  Ultimately, we want to calculate the total volume rise. As long as we know the diameter, starting height, and ending height, we can get a sense of the leavening. As for whether the diameter of the jar actually impacts the ability of microbes to colonize the starter: we dont know! But its a really interesting question and really gets as some of the basic ecological principles of dispersal and colonization.  My hunch is that a very wide jar would mean a very thin layer of starter and therefor a greater surface area exposed to air. In addition to being more accessible to colonizing microbes, it might also impact the ratio of anarobes (the kinds of microbes that dont require oxygen) to aerobes (those that require oxygen, and would therefore only survive on the surface).