You have to use vmware standalone convertor which is a free tool, so that you can perform v2v and shrink the disk size to 25 Gb or as required.when converting. its can be performed as a hot convertion and once the machine is confirgure, shutdown the 50GB machine and power on the 25 Gb machine.This is the best way.
You can download vmware convertor from the below link
Award point for the useful and correct answer by clicking the below tab
Disk on VM is THICK provisioned, if you wanna easly shrink it you should provision VMs with THIN disks. You have 3 workarounds: