use
panelHeight properti on Combo (because combobox is Dependencies on Combo)
<input class="easyui-combobox"
name="language"
data-options="
url:'combobox_data1.json',
method:'get',
valueField:'id',
textField:'text',
multiple:true,
panelHeight:'auto'
">
or
<input class="easyui-combobox"
name="language"
data-options="
url:'combobox_data1.json',
method:'get',
valueField:'id',
textField:'text',
multiple:true,
panelHeight:500
">