First of all... Think in a 120hz monitor...
Second, i dont know much about nvidia gpu, but i think yours is great, i better think about to upgrade to a SSD for OS and DCS... You will get a great performance change in your system.
About the size i'm in the same question... Hehe
I think its more a personal choice, but think that most of the monitors has native 1080x1920 resolution... So if you have a smaller screen, then each pixel will be smaller
Good luck