1920x1080, currently on both a big wall-mounted TV and on a desktop. The TV is rough reading on default scaling settings in Windows, but at 175% I can play perfectly fine. That's what the scaling slider is there for.
Well, I spoke too soon. I finally finished banishing Windows from the rest of my computers, and Fedora (Gnome/Wayland) doesn't scale Dwarf Fortress at all. There's a "text scale" setting in Gnome Tweak that works beautifully for the purpose, but doesn't affect DF. So even I'll be using 20x20 at times. I can't escape!
Along those lines, I've whipped up a simple script that iterates over every tileset in my collection, automatially dices it up into individual tiles, doubles those tiles in size using xbrz, and then stitches them together again. With this, I won't need to support 20x20 at all, it'll just be generated by the script. Splitting it up into component tiles prevents the solid tiles from "bleeding" over into their neighbours. It has a blacklist for tiles 176, 177, 178, 219, 220, 221, 222, 223: even xbrz can't handle these tiles appropriately, so they're doubled separately with a simple "box" or "point" algorithm.
I think some people prefer this approach anyway: the "4chan" variant that still gets circulated is proof that many people prefer the machine scaled version to my manually doubled version, and this lets me keep everything available at double-size with a consistent look and make changes, without me needing to support any of it. This will make only 12 tilesets I need to support in the next version, even with a new tileset in the collection. That makes me happy. Adding the blackletter variant and giving it custom, ornate walls will only result in 2 new tilesets (dwarf letter and normal), where before it'd result in 16 (10x10 sized, 20x20 sized, dwarf letters, normal, and four wall types for each). As mentioned, solid straight walls and diagonal hollow walls won't be supported going forward.
Compare these two tilesets:
As mentioned, I didn't need to open an image editor even once for the one on the left, I just ran my script.
With this, release of the next version will happen soonish. I've redrawn Moscow's walls, so the tetris look is gone but ornate, diagonal are in for two sets now. Even with the new script, the fancy tilesets might just be crudely doubled. It depends on how well xbrz deals with the fine details.