Hi,
The size of the element in the scene window is irrelevant. Only the game window is relevant. When working with UI elements, try to keep the game window open or take a look at it on a regular basis to see if you get the desired results.
Furthermore, there is a slider in the game window. Make sure it is set to 1. Otherwise, you might see a scaled down version of your game.
If you still need to scale the UI elements for some reason, use the Canvas Scaler component.
https://docs.unity3d.com/Packages/com.unity.ugui@1.0/manual/script-CanvasScaler.html
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