How to get the attached disk size on FreeBSD
diskinfo -v ada0 if you just want to see disk size.
If you want to use this info in some script, you can parse sysctl -n kern.geom.confxml output.
Geometry of each disk can be shown by fdisk ada0 utility.
Disks used for filesystems are shown by mount with no args.
Accessible capacity of the filesystem on a disk can be viewed by df -h <filesystem>