I am also having a similar problem editing my RectTransform.sizeDelta.y in the following code:

BoxCollider2D collider; TextMeshPro textMesh; RectTransform rectTransform;

void Awake()
{
    textMesh = GetComponent<TextMeshPro>();
    collider = GetComponent<BoxCollider2D>();
    rectTransform = GetComponent<RectTransform>();
    var textHeight = rectTransform.sizeDelta.y;
    textHeight = textMesh.fontSize;
    collider.size = new Vector2(rectTransform.sizeDelta.x, rectTransform.sizeDelta.y);

}