Tall v Wide is dead. Long live Tall v Wide.
Civ VI just doesn’t have the equivalent of a 4 City Tradition Meta, so Tall v Wide in a Civ V sense is meaningless. And because of Districts and other things, the conversation is more complicated than just replacing TvW with “lots of cities or not too many”. Instead, you have multiple decisions like lots of cities v not too many; a few big high pop cities and lots of small ones, or just lots of pop 4 cities or lots of pop 10 cities; settle or capture cities etc.
The game looks like it was designed for players to have a few really big cities (providing lots of production) and lots of small ones (providing trade routes and gold). If you play according to that design, then your “few big cities” can be very powerful indeed. Build IZs and Aqueducts / Dams, run trade routes from those Cities to Allies using Wisselbank, and build Wonders with +% modifiers and or place high level Governors, and you’ll have a few Cities with heaps of production and growth (and growth means more pop to work more tiles for even more production), and potentially also lots of culture, science, gold and or faith depending on Wonders and Governors. (You’re also probably better having those small Cities being Coastals, because Coastals have more gold overall, and you can get +% discount building Harbours.)
The problem is that, by the time you grow these Big Core Cities and have all the Districts and Wonders and Governors and Trade Routes, there’s not a lot of time left in the game (YMMV) and so little opportunity for ROI. There’s also only so much stuff to build with Production, although there are late game Wonders, the GDR, Scored Competitions and - if all else fails - projects.
However, because of Rationalism, regardless of the original design intention, you’re quite violently pushed to general have just lots of Pop 10 Cities. You don’t even then need any high pop cities, because all your 10 Pop Cities already give you all the Science you need, you get tonnes of gold from Trade (direct and trade routes), so can buy any buildings you need in all your Cities, and anything else you need you can just chop in. So, no “per turn production” required beyond the early turns in a few initial Cities.
So. Are Big Cities viable and or useful? Yes. A few Big Cities and no other Cities works okay; but a few Big Cities and lots of small Cities providing Trade Routes and Gold works much better.
Can you just have (say) Four Big Cities and win? Yes, you can, in the sense that it’s viable. If you do that, there are some advantages of sorts - you can focus on getting Wonders and levelling Governors, you won’t need to move Governors as much, you can take Audience Chambers, and you can focus sooner on just rocketing towards your Victory Condition. But these upsides generally aren’t as good as what you’re giving up by having more Cities.
Are Big Cities optimal? No, lots of Pop 10 Cities and Choppity-Chop is optimal. Just Four Cities, or even just one City, is the least optimal approach, although (1) I guess it’s conceivable that four cities is optimal if you couldn’t expand for some reason (ie because it’s your only option), but you can always expand providing you’re willing to war or travel far enough, and (2) I haven’t experimented with this much myself, but it seems for the reasons others have mentioned you maybe don’t need as many Cities for CV or RV (ie the additional Cities aren’t a negative and or a small empire isn’t inherently better per se, instead you just maybe don’t need so many Cities for these victories and so might be better not spending resources getting more Cities instead of just prosecuting your RV or earning Tourism for you CV).
Should Cities be close or spread out? Depends on how you’re playing the game. If you’re going “a few big cities, lots of small cities”, then ideally the big Cities should be close so they can get better District Adjacencies, particularly from Aqueducts, Dams and Gov Plaza. For your other Cities, I don’t think it much matters. If you’re playing lots of Pop 10 Cities, then again I don’t think it much matters - you just need them to have a Campus and maybe Holy Site and get them to Pop 10; you also need resources to Chop. But I guess in principle closer is better because you can fit in even more Cities, and more Cities is the name of the game.
Hopefully that’s helpful and I haven’t just re-hashed other people’s stuff. I’m sure others will point out if my knowledge is out of date or wrong headed.