Do you mean in the options panel?
We used to show them next to the sliders, but removed it. There isn't really any concept of an "accurate size" other than what the cursor size algorithm uses, and there were some pretty horrible layout issues too.
Basically what you're seeing is a nominal size only, and it can be affected by a lot of other things. It's OK for simple painting brushes or pixel art brushes, but for anything more complicated, I think users are going to be disappointed if they expect accuracy. Am I wrong?
Other possibilities: on the screen (near the cursor?) in a disappearing bubble when you change size. In the status bar at the bottom, also as a temporary message. I like these approaches better, since we might be able to announce brush opacity changes too. Transparency is hard to intuit, and there's no feedback in the cursor. Weirdly, announcing "Opacity: 27%" seems like it would help.