How to reduce parallels disk size

I assigned 128GB to parallels disk when I installed my virtual Windows system. However I realised that I don't need so much space for Windows and I'm running out of Mac OS storage.
So may I know how to reduce disk size from 128G to maybe 64G???
p.s I tried the setting in Hardware/Hard Disk 1/Edit. However the size option range is from 128G to 2TB.

 

Hi, did you try to change size in the upper box, I mean just enter new size (not moving cursor)?