The new floating action button should be 56dp x 56dp and the icon inside it should
As FAB is like ImageView so You can use scaleType attribute to change the icon size likewise ImageView. Default scaleType for a FAB is fitCenter. You can use center and centerInside to make icon small and large respectively.
All values for scaleType attribute are - center, centerCrop, centerInside, fitCenter, fitEnd, fitStart, fitXY and matrix.
See https://developer.android.com/reference/android/widget/ImageView.ScaleType.html for more details.