$25B is the point that Warren has been targeting for a few months now, not just for banking-related services. She says that at $25B, the company is sufficiently large that they control a significant portion of whatever industry they are as well as the economy and people's lives. The goal is to target companies with too much influence and reign them in with regulations and/or breakup. Again, this number is being used for any industry, not just banking and tech (though tech does seem to be the focus at the moment).