Okay, @Temple. Then please disregard my reply as I was answering a different question.
We do not have this documented here. Documentation would belong here btw:
The generic answer is "same as with any (KVM) (Debian using)
qemu-img mentioned by @Ego may be one part, however also within the operating system you would have to update the partition table / repartition to make actually use the newly gained free space.
From https://www.whonix.org/wiki/Grow_Virtual_Harddisk#VirtualBox read from:
(10) Until now we have only grew the physical size, we haven't changed the filesystem. Shut down again.
Not the most simple task until we get this documented.
Depending on your task you might also get your task done by using a bitcoin thin client such as electrum.
Btw also regard: