Which operating system and version of photoshop are you using?
What about cursors for other tools?
Do they have odd shapes as well?
Using windows 7 ultimate 64 bit Os and cs 6 photoshop version.. ya not only to the brush tool, this problem was occurred to other tools also stamp,dodge,eraser,sponge,smudge, blur and spot healing brush tool. when I'm increasing or decreasing these tools these outlines are changed as very odd and formed as four ovals and sometimes its shows four tiny brushes..when I'm clicking some other options on tools, it didn't show the cursor, shows four tiny brush shapes only on the screen..
Two causes come to mind:
1. The windows 7 dpi scaling is affecting the tool cursors.
Try a setting of 100% or if you need a larger setting, try 125% or 149%
2. Your video card (gpu) drivers are the cause.
Have you updated the video card drivers to the latest versions from the video card makers website?
(not through windows update or device drivers update)