I would choose the sink size desired and let that dictate the base cabinet size needed to support it. I think a 17" undermount rectangle sink is common for a 24" vanity base. You don't mention if the vanity is stand alone or in an alcove but I'm sure you're considering placement of the sink base with room on the sides for countertop, and what size drawers remain. For 57" perhaps a 24" sink base with a 15" bank of drawers on each side.

Thank you @bearbev and @JAN MOYER for your comments, and sorry I left out essential information. I added the floorplan to the above. The remodel will be moving plumbing along the wall and shifting it over as necessary.

Most of the sinks I have seen would fit into a 24 inch cabinet, so I'm not sure why a 30 inch cabinet would be necessary. I am wondering if there was a reason for a cabinet sized that large.

Well

The first issue is clearance.

What is the actual potty? You don't even meet code for two full feet feet in front of it with 50" for bath depth. and 15" to a solid anything from center is bare minimum.

The sink is the least of the issue : )

Yes, you're right about the toilet. But this is how the bathroom was originally built. I guess a round toilet would fit. I can give more space to the right of the toilet, but swapping in a toilet is easier than moving the drain for this project.

There is virtually NO lav sink that won't under mount in a 24 inch sink base.

A sink is nothing more than the vessel to the drain for water. You go to Kohler and pick what you want.

You also configure the vanity you want. Personally, I'd enlarge the shower by 6 inches and shrink the vanity to 48 inches. Or buy a 54 inch v or copy one you like

We are having our vanities built by our cabinetmaker (the price really wasn't much different than that of a high quality store one) and we are using a 24" surface mount sink in a 48" vanity, with 16" drawers on either side and a single door in the center. I wasn't happy with the storage on many of the vanities commercially available.