ElevatedButton

ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(shape: StadiumBorder()),
)

---

ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(12), // <-- Radius
    ),
  ),
)

---

ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(
    shape: CircleBorder(),
    padding: EdgeInsets.all(24),
  ),
)

---

ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(
    shape: BeveledRectangleBorder(
      borderRadius: BorderRadius.circular(12)
    ),
  ),
)

Referensi

• StackOverFlow: Create a rounded button / button with border-radius in Flutter