What displays will my iMac (17,1) support?

I am looking to add a 2nd display to my iMac (17,1), but not sure what is the best display or what is the best technical specs it will support. I tried using Duet Display with my iPad Pro, but that doesn't provide enough screen size and the lag when I move the pointer/mouse is less than desirable. So I am going to invest in a 2nd display. Just need some advice to point me in the right direction.

Thanks.