OS X 10.11.5
virtualbox 5.1 beta 3 r108118
On the File location and size window, the disk size changes as you move the slider, but the slider just wobbles back and forth in the same area at the beginning and does not move as you slide to the right to increase the disk size.
[Fixed] Disk size slider doesn't move
Re: Disk size slider doesn't move
Thank you for noticing it! Will be fixed in subsequent release.