Increasing the size of disk is shown on VirtualBox platform.
Increasing the size of disk on other hypervisors, like VMWare ESXi and other, is different, because it have another functional.
Virtualbox platform was installed on Ubuntu 16.04.

Look at our disk in system.

# gpart show

Poweroff server.

Go to directory in Ubuntu, where is image of virtual machine.

Look information of this image with VirtualBox built-in program.

# vboxmanage showhdinfo f5s.vdi

Resize image.

# vboxmanage modifyhd f5s.vdi --resize 10000

Check image again.
# vboxmanage showhdinfo f5s.vdi

Start server and booi in Single User Mode.
Look at our disk.

# gpart show

Size is changed, but we have CORRUPT status.
Correct it.
# gpart recover ada0

Delete swap.

# gpart delete -i 3 ada0

Resize partition.

# gpart resize -i 2 -s 6G ada0

Check it.

# gpart show -p

Growup file system.

# growfs /dev/ada0p2

Check disk with df.

# df -h

Create swap.

# gpart add -t freebsd-swap -s 1G ada0

Reboot server.

# reboot