When ProjectionType = ptPerspective, then PerspectiveAngles specify angles of view (horizontal and vertical), in degrees. When ProjectionType = ptOrthographic, then OrthoDimensions specify dimensions of ortho window (in the order: -X, -Y, +X, +Y, just like X3D OrthoViewpoint.fieldOfView).
Note that ProjectionFar may be ZFarInfinity, which means that no far clipping plane is used. For example, shadow volumes require this.
If you really need to know "what would be projection far, if it could not be infinite" look at ProjectionFarFinite. ProjectionFarFinite is calculated just like ProjectionFar (looking at scene size, NavigationInfo.visibilityLimit and such), except it's never changed to be ZFarInfinity.