FAB.Group should also have a `size` prop similar to FAB

**Is your feature request related to a problem? Please describe.**

The FAB component has a `size` and `customSize` prop allowing to modify its size. So do the individual actions within a `FAB.Group`, however, from what I see the FAB.Group itself has no way of being resized.

**Describe the solution you'd like**
Similar props as `FAB` allowing to use predefined small/medium/large sizes or custom ones!