All the better undermount sinks come with a CAD file for the fabricator to make the 'perfect' hole for that particular sink. If they use that to cut the hole, there will be a nice, even, consistent lineup of the sink and the hole. Done wrong, there will either be a big stone overlap of the sink or the mounting flange will partially show. Now, can you get one to fit? Probably. Will it look as good as it was designed to? Probably not. Will you care if the existing sink drives you to change it? Maybe. Sinks come in various shapes - rectangular, square, D-shaped, small bowl and big bowl in a combo, big, small, big combo, and probably others. Given the depth of a standard counter, there are limits on how big a sink can be, but someones rough size doesn't dictate the actual bowl size, only the flange size. It's a crap shoot as to what you can find that will fit AND look good.