Thank you for contacting Syncfusion Support.Query #1
:" I've tried tweaking the colors, margins and padding but wasn't able to change the FontSize?”
We can change font size of the SfTreeNavigatorItem by using FontSize property of the SfTreeNavigator. Pease refer the below attached sample
Sample: SfTreeNavigatorUWPSampleQuery #2
: "how to change the size of the panel that holds the items when the dropdown button is clicked?"
Could you please provide more information like whether you need to change the height of the SfTreeNavigator based on the items present in it after clicking the arrow icon. This would help us to proceed further on this.