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!

Start by comparing the FAB.Group implementation with the existing FAB size and customSize props, then inspect the individual action sizing behavior described in the issue. Done means FAB.Group accepts the requested predefined and custom sizing options and the component's tests cover the resulting sizes.

Written by the indexing model from the issue text.