complete set of commands involved

preparation with Parallels Desktop itself

first close the machine down if not done already

second got to that machines hardware settings pane

third reveal advanced settings, then click properties

resize to fit your needs (probably you want a backup before :)

click apply

restart the machine

closing act in running machine

open terminal.app and issue the following command:

$ sudo diskutil resizevolume /dev/disk0s2 R

this will move the recovery disk (which is somewhat limited in this regard anyway) to the end of the disk and resize the volume up to the reminding space.