Run the df -h command to get the availaible space in your system:
df -h
The above command will give you the output like this:
Filesystem Size Used Avail Use% Mounted on
/dev/root 7.7G 7.7G 0 100% /
devtmpfs 3.9G 0 3.9G 0% /dev
tmpfs 3.9G 4.0K 3.9G 1% /dev/shm
tmpfs 795M 79M 717M 10% /run
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 3.9G 0 3.9G 0% /sys/fs/cgroup
In the above output you can see that that disk size is around 8 GB.
Run the lsblk command to list the information about all the available block devices on the disk:
lsblk
The above command will give you the output like this:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop4 7:4 0 61.9M 1 loop /snap/core20/1405
loop5 7:5 0 61.9M 1 loop /snap/core20/1242
loop6 7:6 0 43.6M 1 loop /snap/snapd/15177
loop7 7:7 0 55.5M 1 loop /snap/core18/2253
loop8 7:8 0 55.5M 1 loop /snap/core18/2344
xvda 202:0 0 8G 0 disk
└─xvda1 202:1 0 8G 0 part /
The last two lines will give you the indicator of the hard disk size on your Ubuntu instance. In the above case it is 8GB.