You haven't provided enough information here. To me this question is answered not by the vanity, but by the sinks assuming the sink(s) are centered on the vanity. Is it s single sink or a double sink? If so, I would get light(s) that match to the width of the sink area(s). If it's a situation where it's one sink that is offset on the vanity, then I would go by the width of the vanity and have the fixture within the length of the vanity.

It is a single vanity 30". The sink is centered. The counter top is 32". You think a 28" light is too large?