I Recently started learning Flash and due to my nature of looking at wrong places, I was unable to figure out how to increase the size or stroke size of my brush. Well, this is not my fault Completely. Adobe Guys have made it a little confusing for beginners as well.
For Example, When you click on the Brush Tool from the Toolbar, this thing shows up on your right.
The Stroke Slider is Frozen and one can not change its value. This is really irritating. But, this is not what we are looking for. To Change the Brush size Look at the bottom of toolbar. The last two options are ‘Brush Size’ & ‘Brush Shape’.
Here, you can select the desired Brush Size and Shape. Flash CS6 has limited the Maximum Brush Size, and you can not change it beyond an extent. But, I hope you found what you were looking for.