When I re-size the brush tool, selection tool, stamp tool, and others with the "[" and "]" it has problems and shows some wonky shapes or even multiple of the tools and I have to guess where It is on the image and the size. I have PS CS5 on my PC laptop.
Normally, problems like that are due to bugs in the video card driver. Updating the driver from the GPU maker's web site (not Microsoft) usually solves the problem. Sometimes it's a new bug in the driver, and the GPU maker needs to be informed so they can fix it.