Multi-Select List customization?

I created a Multi-Select List in a custom task presentation to display the content of an enumeration. I have around 25 items in this enumaration.

1) How can I control the height of the Multi-Select List so that it will not show the all 25 item in a very long Multi-Select but have a enough height to display say 5 item and have a scroll bar next to it? Both in native and web client presentations. Feels like there must be a key-value property to set this.

2) How can I enforce the user to select at least one from the Multi-Select List when they create my custom work item that has this Multi-Select List?

Thanks,
Aydin