This way:
    <levelsize>
      <size width="1200" height="600" />
    </levelsize>

    or:
    <levelsize>
      <size>
        <width>1200</width>
        <height>600</height>
      </size>
    </levelsize>

    or:
    <levelsize>
      <width>1200</width>
      <height>600</height>
    </levelsize>

    or alternativly:
    <levelsize width="1200" heigth="600" />

    or maybe just:
    <level-width>1200</level-width>
    <level-height>600</level-height>