So, how much will it be Samsung Galaxy S10 +? This video (Posted by @IceUniverse) should help you give a good idea of the dimensions. In short, it's the same size as the Galaxy S9 +.
You can say that this is because the case for the upcoming flagship fits with the S9 + (and the note 9 is too big). The S9 + has a 6.2 "18.5: 9 display and measures 158.1 x 73.8 x 8.5 mm.
The Galaxy S10 + is expected to have a 6.4 "display with a higher ratio and a dual camera with just a few holes. Here you can see CAD rendered based on the phone – they give it its dimensions like 157.5 x 75.0 x 7, 8 mm.
Part of what Samsung allowed to bump on the screen of the body ratio is a smaller chin. As you can see in the video, Oppo Find X also fits into the S10 + case and this phone has a 6.42 "19.5: 9 screen (and a bit on the way to frames).