[new feature] Stepper: add button-size prop (#3714)

src/stepper/README.md

@@ -81,6 +81,16 @@ export default {
 }
 ```
 
+### Custom Size
+
+```html
+<van-stepper
+  v-model="value"
+  input-width="40px"
+  button-size="32px"
+/>
+```
+
 ## API
 
 ### Props
@@ -95,7 +105,8 @@ export default {
 | disabled | Disable value change | `Boolean` | `false` |
 | disable-input | Disable input | `Boolean` | `false` |
 | async-change | Whether to enable async change | `Boolean` | `false` | - |
-| input-width | Input width | `String | Number` | `30px` |
+| input-width | Input width | `String | Number` | `32px` |
+| button-size | Button size | `String | Number` | `28px` |
 
 ### Events