They're roughly the same size although the X does have a much larger screen. However they're removing the reachability feature which means that a good chunk of that larger screen will be difficult to use one handed, which won't be true of the 8.
The iPhone 8 does not have dual cameras, which for some people is the key reason for going with a Plus-sized model. I hate how big my 7 Plus is, but I bought it for the cameras. I look forward to having a smaller dual-camera phone — either the X or a subsequent model.
The iPhone 8 Plus is roughly the same size as the X, not the iPhone 8.
No. The iPhone 8 Plus screen size is roughly comparable to the iPhone X, but since the bezels are gone, the iPhone X is significantly smaller in physical size.