Hi everybody ! 
I’m a beginner and I’m folowing TIC TAC TOE tutorial… But my issues happen very often. When I enter play mode I don’t see all the scene… it’s better when I’m on maximise mode but I still don’t see a button.
whereas I have same pixel size for my gameObject… And all are in the same canvas. 
I hope somebody can help 
Sounds like maybe it’s offset from the anchor too far to the side, thus offscreen?
If for example you want the button on the right top corner (or close to), better to anchor it there and then move it inward/downward a little bit.
Later, if you still have issues, you could try the canvas scaler component.
If you search for “UI multiple resolutions” there’s a Unity post on the topic for that.
If that doesn’t help your situation, could you explain a bit more, maybe?
I often experience annoying problem with game tab scaling - it’s often automically sets to half slider, make sure you don’t scale it.
Also here you have some useful texts about positioning you UI:
Redirecting to latest version of com.unity.ugui
Redirecting to latest version of com.unity.ugui
Redirecting to latest version of com.unity.ugui
Unity UI Tutorial: Canvas and Scalability - Cat Trap Studios
Good point, sometimes I forget to think about the “window scaling”. Make sure that’s not on lol