Linux does not (always) automatically detect size changes on the virtual disk, nor does it resize the partition.
For the first step to happen, you need to either reboot the server, or execute "echo 1 > /sys/class/scsi_device/x:x:x:x/device/rescan" for the appropriate device on the command line. (if unsure, just do it for all - nothing bad will happen)
After that you need to use fdisk or gparted to delete the existing partion and recreate it with the new size. (i.e. change the end block of the partition)
If this is done, you may need to inform the Linux kernel of these changed partition sizes - this can be done via "partprobe /dev/sdX" (sdX = virtual disk in your server, can also be vda or vdb, depending on your setup)
And finally you can increase the filesystem size by executing "resize2fs /dev/sda1" (or sdb1, or vda1..)
To reflect the new size in Plesk, you need to either wait or manually execute a rescan for changed hardware