If you don't need the extra storage (being a powder room), I think I'd go with your contractor's recommendation and purchase a 36" vanity with mirror and keep the sink in same location. Use the open space for decorative items.

Using a 36" vanity saves additional cost because you won't need to relocate the sink plumbing.

@krugg reflections, do you know where I can find something like this? I love these options

These particular sinks are from stoneforest, they are beautiful but a bit pricey. But I know there are other companies with similar looks maybe for a more competitive price, can't remember top of my head.

Perhaps search Console sinks, and add some filters like "marble top" or modern or traditional depending on which direction you want to take.

Nice suggestion by Krugg. For a powder room, go dramatic with the sink.

These folks sell just the stands and you can have a top and sink cut to fit. http://www.sinklegs.com/dealer-locator

Hi. How about an entirely floating vanity. You have the perfect situation for it, with a wall on each side :) So easy to clean, bonus!

There are endless options for vanity width and layout - contact a local custom cabinet builder! I do like the idea of the dramatic sink only, but even in a powder room, a little bit of storage is nice.

Restoration Hardware also has some similar to Stoneforest

you can right click any image and copy the link. Then go to google image search and paste it in. Google will return all the places where visually similar images appear.

Google images search pulls up the links from above

http://www.sinklegs.com/

and stoneforest

https://stoneforest.com/products/251-renaissance-console-top