Currently, the RadMultiColumnComboBox has smaller height than the AutoCompleteBox or the RadComboBox for example. 

A possible workaround can be using the following styles: 

<style>
    .RadMultiColumnComboBox .k-dropdown-wrap {
        padding: 4px 12px;
    }

    .RadMultiColumnComboBox .k-select {
        width: 1.42857143em;
        height: 1.42857143em;
        position: absolute;
        box-sizing: content-box;
        top: 0;
        bottom: 0;
        right: 0;
        left: auto;
        padding: 6px;
    }
</style>

We have just upgraded to the latest asp.net ajax version and excited about the new control 'RadMultiColumnComboBox', but we could not figure out how we can allow multiple selections.

Please let us know

Thanks,

The new MultiColumnComboBox control does not have any server side events.  Can you provide the same server side event that you have for the ComboBox?