We have Toto Rendevouz (17" x 14" oval undermount) sinks in each of our masterbath vanities. I really like these sinks so we are putting two of them in our son's 85" vanity. I think they look large and well-proportioned and I personally wouldn't want anything larger. I've seen pictures of bathrooms with larger sinks and some of them do look nice but, for me, sinks are more of a functional item and less of a design feature.

What is the front to back dimension of the sinks that you are considering? That will have more bearing on whether it will fit in your 21" deep vanity. If the sink that you really want will fit your vanity, then I'd go for it because whether it is worth the extra expense and possible difficulty is a personal decision.

Good to know that you like your Toto 17x14 oval, as this is basically our other choice. We are deciding between the Toto LT569 (17" x 14") and the Toto LT546G (19" x 13") - I don't know the style name. The vanity depth is 21 and the cabinet width is 21 also, which means this is one of the only 19 that would fit as it is only 15" front to back and also that we would need to cut into the side of the cabinet in order to make it fit. It sounds nuts but I am such a mess with the sink -- I don't know how but water seems to always end up everywhere so I thought a bigger sink could help contain my mess!

I'd suggest checking your product specs carefully because, unless your 21" is an interior measurement, I wonder if a 19" sink would fit, even if you're prepared to cut the side of the cabinet. We had a close call on our son's vanity because we had asked our Amish cabinet maker for 22" interior width sink bases and he mistakenly made 22" wide cabinets. We fit our 17" sinks with little room to spare.

One thing to consider with a 19 x 13 as compared to a 17 x 14 is where your spout will reach. You may find that the extra room you gain laterally may not compensate if the sink/faucet makes it feel shallow as you lean over the sink. I don't know, I've never had an oblong sink, just something to think about.

In case it helps, here is a photo of our masterbath faucet over the 17 x 14 oval sink:

You raise a very good point about the faucet, I hadn't thought about that. I think sticking with the standard 17" sounds like the smartest idea. Thanks for all of your help!