Place the following expression on the shape layer > contents > “rectangle” > rectangle path > size

---

margin = 50;

text_layer = thisComp.layer(“test”) //pick whip to desired text layer

text_width = text_layer.sourceRectAtTime().width;
text_height = text_layer.sourceRectAtTime().height;

shape_width = text_width + margin*2;
shape_height = text_height + margin*2;

[shape_width, shape_height]