Nothing should change the dimensions of your sprite by anchoring. Make sure you have the latest version.
The only useful thing to know about the new system is that you can anchor to Panels, Cameras, Widgets, and Transforms. How it behaves depends on what component is found on the "Target".
Cameras, panels and widgets all have width and height, so you can anchor to their sides, corners and center.
Transforms (game objects with nothing on them) don't have any width and height, so when you anchor to a raw game object, there is no "left", "right", etc to choose from. You only get 4 offset values. So if the transform happens to be positioned at XY on the screen, the 4 values are simply added to that position to produce the final widget's coordinates.
When you want to anchor something to the top left corner of the screen, you should either specify the Target to be the Panel (which is by default on UIRoot now), or the Camera (your UI camera). Otherwise you're anchoring relative to a completely different object.
Lastly, there is never any need to set the 4 values manually. Simply resize your widget in the scene view like you normally would if it wasn't anchored.