(1) GetCanvasHeight, GetCanvasWidth
You can imagine these are the height and width of the bounding box of all the pages arranged in PDFViewWPF. In continuous presentation mode, it includes all the pages either visible or invisible (so it typically has a large height value). In single page presentation mode, it only includes the current visible page.
(2) GetExtendedCanvasHeight, GetExtendedCanvasWidth
If you are zoomed out enough, all the pages are visible simultaneously and in this case GetCanvasHeight() is actually smaller than the height of the viewport. GetExtendedCanvasHeight () simply returns the the value whichever is greater.
(3) GetViewHeight, GetViewWidth
These two functions simply returns the height and width of the internal buffer for the viewport. So yes, they should be the same with the dimensions of the viewport.