Rect Transform, adjusting size via width/height, or scale?

Questions & Answers
1

This feels like a simple question, but the documentation wasn’t too helpful.

A Rect Transform has both the properties, Width/Height, and Scale, X/Y.

Changing these values both resize the rect transform.

So, which one is the “correct” way of making the UI element the size you want?

2

Definitely width and height.
If you, for example, have a UI Image and with a sliced background, changing width and height will not distort the image. Changing the scale will distort the image.
Here’s how it looks with changed width:
120515-width-height.png

Here’s how it looks with changed scale:
120516-scale.png