message: docs(Loading): add size prop demo
diff: @@ -25,6 +25,13 @@ Vue.use(Loading);
  <van-loading type="spinner" color="#1989fa" />
  ```
  +### Size
  +
  +```html
  +<van-loading size="24" />
  +<van-loading type="spinner" size="24px" />
  +```
  +
  ### Text
  ```html
  