For better understanding, resizing the game window was NOT possible prior to SDL. While one no longer has to calculate gridsize for a character tileset they are using (thanks to the SDL version) it can produce some questionable results if you don't choose a character tileset size that was meant for the game resolution you intend to use (e.g. with the minimum gridsize of 80x25 a 32x32 pixel set needs a minimum resolution of 2560x800).
The default settings add black space if your set was designed for a lower resolution than the resolution you are using or squishes the tiles down if it was designed for a larger resolution. I believe the ugly "zooming" people experience is a side effect of this visual compromise.
Gridsize still matters if you want the character tileset to display as intended; resizing the the window via the SDL version alters that gridsize.
As Taffer mentioned and without the need to worry about gridsize at all because of the SDL version, what character tileset size you want depends on what you are looking for. In addition to the questions Taffer asked, do you want more tiles on the screen at the expense of readability or do you want readability at the expense of how many tiles are displayed? Are you running the game in fullscreen mode or just maximizing the window?
Assuming your entire resolution is 1920x1080 the largest character tileset you can run fullscreen without distortion would be a 24x24 or 24x32 pixel tileset (square or non-square respectively). Anything larger won't display as intended. Using a lower pixel size (e.g. 12x12) will increase your real estate (obviously). When using a smaller tileset, if you stick with the same aspect ratio as the largest tileset's gridsize that you are capable of running you can eliminate the black space. Although, as Taffer pointed out, the black space is a non issue - you're just not using the maximum amount of real estate that you could be. The distortion is what you probably want to avoid.
I imagine the black space also buffers the smaller character tileset from distortion if you're just maximizing the window rather than using fullscreen mode. That's just a guess, since I only use 12x12 and 24x24 sets to avoid black space; I resize my resolution without scaling to play DF meaning there's already enough black space and I don't want extra. Heh.
Thanks for any education you can give this hairless ape
I'm not sure if that was the education the "hairless ape" wanted - but I hope it helps.