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!

首先将 FAB.Group 的实现与现有 FAB 的 size 和 customSize props 进行比较，然后检查 issue 中描述的各个 action 的尺寸行为。当 FAB.Group 接受所请求的预定义和自定义尺寸选项，并且组件的测试覆盖由此产生的尺寸时，即表示完成。

由索引模型根据 Issue 内容生成。