If you’re wondering about the relationship of brushstrokes in Illustrator to the stroke weight measurement you find in the Stroke Panel, I’ve created a graphic to illustrate the concept.
Changing the stroke weight (in the Stroke Panel) of a path you’ve created with the Brush Tool is like applying a multiplier to the brushstroke. By default, your stroke weight should be set to 1 pt. in the Stroke Panel and then any brush stroke you create will remain true to it’s original stroke size.
Calligraphic brush tips are sized in terms of points. Double-click any Calligraphic brush tip in the Brushes Panel to see it’s options, including it’s stroke size. If you are using a 3 pt. tip and your stroke size is set to 1 point in the Stroke Panel, then your brushstroke will appear at it’s original size, 3 points. But if you set your stroke weight to 2 points in the Stroke Panel, you’ll be adding a multiplier of 2 to your brushstroke, leaving you with a 6 point stroke.
Changing the stroke weight in the Stroke Panel in Illustrator will give you the same results with Art brushes, Scatter brushes or Pattern brushes - you’ll be applying a multiplier to the brushstroke. Art, Scatter and Pattern brushes have sizes expressed in terms of the percentage of the original vector art used to create the brush.
Finally, in Adobe Illustrator CS 5 the new Bristle brush is available, it’s yet another vector-styled stroke that gets applied to a path as you “paint” with the brush tool. The width of a bristle brush is expressed in terms of millimeters and you can change it’s width by double clicking the brush in the Brushes Panel. Like the other brushes I’ve mentioned here, you can instantly double the size of the stroke by changing the stroke width to 2 in the stroke panel, triple it by changing the stroke width to 3, etc.
Particularly with the bristle brush, but also with the other brushes, I think the best practice is to adjust the stroke size on the brush itself, not by adding a stroke panel adjustment to the brush. I say this in particular with the bristle brush, because it creates a lot of complex paths to begin with, which makes files larger and slower, and I might not want the added complexity. But at least understanding the relationship between Illustrator brushes and stroke panel sizing is important for knowing your options when using the brush tool.