uGUI scales your canvas to be the size of your screen or Scene view, but expressed in world units.
It is normal for a canvas to appear very large in comparison to other objects.
You can press F with an object selected to quickly refocus the camera between large and small elements.
Sometimes you want to get the canvas out of the way so you can work on other objects in the scene. See Scene view visibility and Scene view selection to learn how to toggle visiblity/selection for Scene view objects.
If you create a prefab out of a Canvas the prefab environment can be a great way to work on UI in a manner isolated from the scene.
When working on UI it can be difficult to quickly move to select elements that are nested in the UI, causing your hierarchy to become expanded as you select through, or involving manually searching for you object.
I have a tool called NSelection which allows quick selection of any object under the cursor.