Power BI Property pane - Slider control

Yes, this technique isn't really documented. I discovered it by reverse engineering examples created by others.

Begin by creating an integer property objects section of capabities.json.

    "objects": {
        "barchartProperties": {
            "displayName": "Bar Chart Properties",
            "properties": {
                "xAxisFontSize": {
                    "displayName": "X Axis Font Size",
                    "type": { "integer": true }
                }
            }
        }
    }

In settings.ts, define the VisualSettings class with the property based on the number type.

module powerbi.extensibility.visual {

  import DataViewObjectsParser = powerbi.extensibility.utils.dataview.DataViewObjectsParser;

  export class VisualSettings extends DataViewObjectsParser {
    public barchartProperties: BarchartProperties = new BarchartProperties();
  }

  export class BarchartProperties {
    xAxisFontSize: number = 10;
  }

}

Finally, extend the enumerateObjectInstances method in the visual class to add a range of valid values using the validValues property of the visual as shown in the following code.

public enumerateObjectInstances(options: EnumerateVisualObjectInstancesOptions): VisualObjectInstanceEnumeration {

    var visualObjects: VisualObjectInstanceEnumerationObject = <VisualObjectInstanceEnumerationObject>VisualSettings.enumerateObjectInstances(this.settings, options);         

    visualObjects.instances[0].validValues = {
        xAxisFontSize: { numberRange: { min: 10, max: 36 } }
    };         

    return visualObjects;          
}

This allows you to create a slider and explicitly set the min and max values.