Incorrect disk size

[yeraycito]

My mini-pc has a 256 Gigabyte ssd, however Opnsense only recognises 21 gigabytes.

[cookiemonster]

Those 21 GB are the size of the dataset, that is the filesystem. It was probably a value entered at installation time or maybe a suggested size accepted.
A "gpart show" will show you the partition layout. It should show as freebsd-zfs (21 G) for that partition.