Hi and welcome
How do you mean modified?Do you mean increased vmdk size?
yes，i increased the vmdk size ,now i want to modified the size to former.
In general disks grow is a non-distructive process that could expand capacity when disk is online or offline. The reduction process is a distructive operation that could not be executed online and requires other processes like clone data or copy.
If you need to reduce the vmdk, the hard but "pure old" operation is:
1. add new vmkd with the original size
2. boot your VM with a bootable linux distro like Clonezilla
3. copy data from vmdk1 to vmdk2 (obviously format vmdk2 with the same type of vmdk1)
4. remove vmdk1 and reassign vmdk2 scsi position or modify bootloader with the new scsi target
In all cases this procedure is critical and distructive... pay attention!
There are also other ways to do that:
1. v2v conversion: use converter and resize new VM with with the original size
2. Use thin provisioning and remember to shrink with vmkfstools... this is virually reduction, because OS see the extended size.
Hope this could be helpful