You can add this on theme data

ThemeData(
  colorScheme: ColorScheme.fromSeed(seedColor: primaryColor),
  useMaterial3: true,
  filledButtonTheme: const FilledButtonThemeData(
    style: ButtonStyle(
      minimumSize: WidgetStatePropertyAll<Size>(
        Size(double.infinity, 40),
      ),
    ),
  ),
)