In this link, a xpz with the SAP UI5 MultiComboBox implementation in a GeneXus UC2.0 and a sample can be downloaded.
If you want to modify the height of the control or the font size, you can create a Custom class, named Ui5MultiComboBox for example.
In this Class, you can set the following proerties in Custom Properties:
---ui5_input_height: 20px; Set the height of the control to 20px
---sapMFontMediumSize: 10px; Set the Font Size to 10px
Then the MultiComboBoxCustomClass UC property can be seted with the name of this custom class that has been created, following with the sample: