It seems I'm not the only one getting this problem and was told to report it. I had previously installed last Catalyst Beta driver and after updating to 15.7 I got this problem, with all mouse cursors being x2/x3 times bigger. I went to Control Pannel -> Make mouse easier to use and it's set on the minimum size, so it's not due to this.
I'm using a R9 290 on W7 64 bits connected to a Samsung 2333HD via HDMI.
Anyone with a possible solution? I've read about this problem happening before and solving it by updating the graphic BIOS or fixing EDID (link) but I'll pass on that and it has tu be a bug.