How to add new HTML element with inline styles?

I would like to add a new HTML element, with inline styles, around the rte.selection().

With the following example, the HTML is OK, but probably not the JSON part:

    const editorRte = this.editor.RichTextEditor

    // ...

    editorRte.add('fontSize', {
        icon: `
          <select>
            <option value="" disabled selected>Size</option>
            <option value="8px">8 pixels</option>
            <option value="12px">12 pixels</option>
            <option value="16px">16 pixels</option>
            <option value="72px">72 pixels</option>
          </select>
        `,
        event: 'change',
        result: (rte, action) => {
          const selectedValue = action.btn.firstElementChild.value || action.btn.firstChild.value

          rte.insertHTML(`<span style="font-size: ${selectedValue}; line-height: 100%;">${rte.selection()}</span>`)
        }
      })
    },