UI elements use RectTransform.

It controls:

• position
• size
• anchors
• pivot
• scale

Code:

RectTransform rect =
    GetComponent<RectTransform>();

rect.anchoredPosition =
    new Vector2(100f, 50f);

rect.sizeDelta =
    new Vector2(300f, 100f);

Anchors define how the UI relates to its parent. Pivot defines the internal reference point used for position, rotation and scale.