Enlarging the disk in VMWare is the easy part, for that you go into the VM Settings and type in a larger number. (So long as you have space free on your data store, or your using thin provisioning and you are aware of the limitations of over provisioning.)
The bigger issue comes within the OS that your FTP server is running on. What operating system are you planning on running within VMWare?
If its linux server then you can use LVM which allows for on the fly disk expansion.
To do this you 1st enlarge the disk in VMWare then you create an additional disk in the new space, combine them into a single 'volumegroup' then extend the 'logicalvolume' then resize the file system. This can all be done while the system is live and running.
If the FTP daemon is running on a windows server then you want to have 2 partitions/disk and make sure that your FTP share is stored on the 2nd disk. Its not possible to expand the drive/partition marked as 'system' from the install of windows that is running on it.
I hope this makes sense.