So, how large will the Samsung Galaxy S10+ be? This video (posted by @IceUniverse) should help give you a good idea of the dimensions. In short, its the same size as the Galaxy S9+.
You can tell that it is because the case for the upcoming flagship fits snugly on an S9+ (and the Note9 is clearly too big). The S9+ has a 6.2 18.5:9 display and measures 158.1 x 73.8 x 8.5mm.
The Galaxy S10+ is expected to have a 6.4 display with a taller aspect ratio and a dual selfie cam inside a couple of punch holes. You can see CAD-based renders of the phone here they give its dimensions as 157.5 x 75.0 x 7.8mm.
Part of what allowed Samsung to bump up the screen to body ratio is the smaller chin. As you can see in the video, the Oppo Find X also fits into the S10+ case and that phone has a 6.42 19.5:9 screen (and little in the way of bezels).