144Hz is the screen refresh rate. This is a lot more noticiable in old CRT than in LCD screens but as rule of the tumb the higher the better (with a 1060 could get decent framerates but at max you would get 60-90 fps i suppose, 144Hz is a bit of waste), it's more noticiable on fast moving games that need very fast actions like shooters.
1440p is the resolution (2K for 2560 × 1440 or 3440x1440 in a ultrawide screen), in this case the vertical resolution. The higher the more clear the image will be (and the smaller you will see them), its the maximun resolution (2560 × 1440) i would recomend for your GPU (the higher the most resources you need from your GPU) and only in 27'+
The last is the size, for that price range i would go for a 27' but you will have three options there.
- 27' Faster screen (TN) with "worse" color quality
- 27' Slower screen (IPS) with "better" color quality
- A screen with cuts in some areas (size, refresh rate, etc..) that trade them for specific technology like GSync, ultrawide format 21:9 instead 16:9, etc..
It's just a personal choice here.