Hi,
Can you please let us know which version of the app you are running? You can find it under About screen(Win: Help>About <app name> Mac: <app name> ->About <app name>
Couple of things to note:
1. Stroke properties are not applicable for Brush tool. It just produces fills
2. If you are referring to the Brush size controls then they are exposed in PI only from Flash CC 2015(under Fill and Stroke section of Brush Tool PI) and in Animate under Brush Shape section of Brush Tool PI
3. You can use Paint Brush tool in the latest release (renamed as Animate CC) to draw a stroke based brushes - Adobe Animate Help | Working with Paint Brush
Thanks!
Mohan