How is the base virtual disk provisioned? Thick or Thin?
In case of a thick provisioned virtual disk, consolidating a snapshot (with a powered off VM) should not consume additional disk space, i.e. chances are good that consolidation succeeds.
In doubt, run ls -lisa and compare the provisioned size with the consumed disk space (in kB) in column 2. If the sizes match, it's a thick provisioned virtual disk which size should not grow with the consolidation.
That said, it's important to have at least some free disk space on the datastore, which should be the case, if the VM has been powered off, and the swap file - which by default is created with the size of the VM's provisioned RAM - is gone.
In doubt, please post a complete file listing (the output of ls -lisa).
André