Make sure your sprite collection is created with the same settings as the UI sample sprite collection (specifically, ortho size = 1, tk2dCamera = off) - if it is, then you can simply replace the sprites as you've done now, and the sprites will be the same sizes. That should be it.
In terms of workflow, the easiest / best thing to do is to drag the prefabs into the scene as described in the tutorial, and customize them the way you need them for your game, and then create prefabs from these to use in your own project.