This example shows how to set the width of combobox to a percentage of its parent container.
<div class="easyui-panel" style="width:100%;max-width:400px;padding:30px 60px;">
<div style="margin-bottom:20px">
<input class="easyui-combobox" name="language" style="width:100%;" data-options="
url: 'examples/combobox/combobox_data2.json',
method: 'get',
valuefield:'value',
textfield:'text',
groupfield:'group',
label: 'width: 100%',
labelposition: 'top'
">
</div>
<div style="margin-bottom:20px">
<input class="easyui-combobox" name="language" style="width:50%;" data-options="
url: 'examples/combobox/combobox_data2.json',
method: 'get',
valuefield:'value',
textfield:'text',
groupfield:'group',
label: 'width: 50%',
labelposition: 'top'
">
</div>
</div>