See the
Tileset repository on the wiki for some ideas. On my primary home monitor which is 1920 x 1200, I currently use "Yayo's Oreslam", a square 20x20 tileset with windowed settings at WINDOWEDX:95, WINDOWEDY:55 (1900 x 1100, so as to still be able to see other stuff underneath). I've also used "Haowan's" 18x18, which is a hair smaller but looks slightly closer to the classic characters, with good success.
For a true 2560 x 1080 monitor, the equivalent settings for a 20x20 would be WINDOWEDX:127, WINDOWEDY:49 (2540 x 980 window).
For comparison, assuming 20x20 tiles, windowed, 20 px total for left & right margin, 100 px total for top & bottom margin (you can probably squeeze that down to 40 px total, getting up to 3 more vertical characters, depending on your window layout and personal preferences):
* non-square old school (typically 4:3) is 80 x 25 = 2,000 squares
* 1600 x 900 yields 79 x 40 = 3,160 squares (1.78 aka 16:9)
* 1920 x 1080 yields 95 x 49 = 4,655 squares (1.78 aka 16:9)
* 1920 x 1200 yields 95 x 55 = 5,225 squares (1.60 aka 16:10)
* 2560 x 1080 yields 127 x 49 = 6,223 squares (2.37, not quite anamorphic standard 2.39)
* 2560 x 1200 yields 127 x 55 = 6,985 squares (2.13)
* 2560 x 1440 yields 127 x 67 = 8,509 squares (1.78 aka 16:9)
* 2560 x 1600 yields 127 x 75 = 9,525 squares (1.60 aka 16:10)
Personally, for most "practical" use I find the vertical space more useful; my 1920 x 1200 home monitor feels more useful than my 1920 x 1080 laptop more than the simple pixel count would indicate. My work monitor is a 2560 x 1440, and with rare exceptions (mostly really wide spreadsheets) I find the additional height to be what I appreciate. Your use cases and tastes will of course vary; but unless your primary use for the monitor is watching high-ratio, high-res (Blu Ray or better) widescreen cinema at close to native 1.85 or 2.39 ratio, I'd personally be slightly dubious about a monitor that wide but that short. DF with the right settings will handle whatever you get, of course.