Your old monitor is still valid by all measures as long as it works flawlessly. 24" is also a very good size for 1920 x 1080. So if you're happy with it there's nothing to be ashamed of.
However, your video card is capable for much more than what your monitor can display. There's two routes you can choose and even mix to a certain extent.
Route 1: More pixels and size. You've most likely heard about 4K. It literally is as tough for a video card as four of your current monitors. If the image quality of your current one pleases you, a 48" 4K would give the same level of sharpness with quadruplicated details. Small 4K screens are somewhat of a waste of resources since the details in pictures would be too small for you to see. If you want something between 24 and 48 inches, there's resolutions like 2560 x 1440 which according the same math would suit a 30 - 34" screen. Also, there's 30 - 40" ultra wide screens at 3440 x 1440, all of which are easier to the GPU than 4K. Notice that size doesn't matter, the pixel count does.
Route 2: Higher refresh rate. The refresh rate of your monitor is what limits your frame rate. Although some people used to say that everything past 30 FPS is overkill and can't be seen, mostly based on the movie frame rate, it's a fact that in real life there's no frame rate or it's infinite. In either case anything less only gives an illusion of fluency. Back then players with better hardware claimed they could see the difference between 30 and 60 FPS, now it's between 60 and 120 FPS. So the better the equipment get, the less we allow our eyes and brains being fooled.
For a GTX 1070 Ti 4K should play quite nicely at 60 Hz, maybe with some details cut. A 1440p or an ultra wide at 100+ Hz might be nicer for fast paced games.
