Here are the couple of steps you can try.
1. Power off the virtual machine , and remove the .vmdk file ( what you wanna increase from the virtual machine , dont delete remove it )
2. Select any other virtual machine and then go to edit settings and then add a new harddrive , when it asks to create a new one , or use an existing one , point to to the one which we want to increase.
3. Then since it would not be a primary partition on the new virtual machine you can use a diskpart utility to increase the size or merge the unallocated space , then unpresent the harddrive from the virtual machine and add it to the original one with an extended space.
Simple to follow and easy to do.....