The Samsung Galaxy S8+ comes with a 6.2-inch (full-rectangle, 6.1" if you count the rounded corners) QHD+ Super AMOLED display. So it's the same screen design as the LG G6 in terms of design and resolution, but it's even bigger. In fact, it's 6.2" if you consider the full rectangle. It seems that a rounded-corner screen is last word when it comes displays in high-end smartphones. Samsung has designed the Galaxy S8 Plus with minimalist bezel, including the top and bottom parts. This allows Samsung to cramp an even larger screen into the phone. It could have chosen to make the phone smaller, but everyone wants a larger display, as long as it doesn't have a very negative impact on the usability and portability.
This "all-screen" design with curved edges and ultra-slim bezels isn't a new thing, but we can clearly see that this is the design that the leading phone manufacturers have chosen to adapt. Some companies like Xiaomi took it even further with its Mi Mix smartphone, by employing very advanced technology to create an edge-to-edge display that covers almost the entire front side of the phone. The Samsung Galaxy S8+ isn't like that, but it still seems to design and optimized with one-hand usability in mind, like the G6.
The Galaxy S8 plus reminds me of the Huawei Ascend Mate7 and the Oppo R7 Plus that have a 6-inch screen and had relatively slim bezels. There are plenty of phones out there with a 6" or larger displays, but most of them aren't comfortable to hold in one hand and have large chins. The trend pushes to more compact phones while keeping the same screen size. The 18:9 aspect ratio also comes into play because it allows phone manufacturer to create narrower phones while taking advantage of the extra vertical space—like in the LG G6 with its 18:9 aspect ratio screen.
Having a 6.2" screen is great for doing a lot of thing, like playing games, browsing the Internet and watching movies. In fact, everything that you do on the phone is enhanced and well be more comfortable, productive and fun on a larger display.
I've seen the leaked images of the Samsung Galaxy S8 Plus, and that screen looks huge. You just can't mistake it for another screen, you can see it's narrower and longer than the conventional 16:9 displays.
So the Samsung Galaxy S8+ is, based on early leaks (I'm writing this prior to the official announcement), will have a massive screen, but with little impact on portability and usability.