What kind of sink should I put in a super tiny bathroom?

Without moving water lines, your options may be limited to Duravit rectangular with side mounted faucet, or a powder room sized sink. Type in "images of tiny powder room sinks" on Google, to see options. If it would help, you can always mount the sink higher, although you'll have to make some adaptations to fittings. Sink with rounded or oval edges will give you slightly more space.

there was at one time not sure yet available a extra small wall hanging lavatory for use in children's area in day care locations---you may need to contact plumbing fixture manufactures to see if still available----most of those companies have their offerings online as well---may need to Google Plumbing Fixtures Manufactures First to get an up to date list!!! If you are in a large town you may have a dealer in salvage housing items from buildings being torn down, plumbing items are a big part of their business!!!

Here's a cute little sink! https://www.faucetdirect.com/proflo-pf1131p-19-1-2-centerset-lavatory-pedestal-sink-only/p186972?source=msn-pa_186972!c291406401!a1260040454264192!ksmall%20sinks%20for%20small%20bathrooms!me!dc!no&cvosrc=pla.bing.186972&cvo_cid=291406401&cvo_crid=78752558775689&cvo_adgroup=1260040454264192&matchtype=e&cvo_uniqueid=186972&utm_source=pla&utm_medium=bing&utm_term=186972&msclkid=027f4fc5e9b119ba854a14917368b93a

They also have small sinks at Menards. And some have tiny vanities to go with them!

A round bowl toilet and a vessel or pedestal sink may open up the area. Check out tiny bathrooms on Pinterest.

I think most Daycare center have children sinks, so if you want to go that small their available, and new vanities are now countertop heights so you can install the sink higher than the old heights.

Pedistil

Have you asked a plumber about the advisability or cost of moving the toilet closer to the tub? Have you checked any architectural salvage companies for smaller sinks? Since you don't say there is, I'm assuming there is no room to move the sink, further down that wall? Could the door be switched to open into the hall (or whatever is outside the bathroom) rather than into the bathroom?

I would put a small pedestal sink in. They take up very little room