Hi Teemu,
regarding to the Plex Documentation you can do the following:
List Box Height Factor Property
List Box Height Factor Property
Applies to: Combo Boxes
This property specifies the height of a combo box's drop-down list box at run time. The property has a default value of '4' and gives the user the ability to increase size of the drop-down list box at run time. For example, a value of '4' shows about 4 rows and a value of '8' shows about 10 rows, depending on the size of the font selected for the combo box's edit control. The value of '4' was the original, hard-coded value before the List Box Height Factor property was introduced.
This setting does not affect the combo box's design time appearance. Some trial and error testing may be required to achieve the desired result since the run-time size is determined by a combination of the size of the font chosen at design time and this height factor.
The List Box Height Factor property is inherited through the Control Defaults.