I've increased the size of my VDI using
VBoxManage modifyhd myHDD.vdi --resize 50000but I can't make the guest recognize and use the new space.
It looks like VirtualBox is seeing the increase in size (marked by the 48.83GB in your screenshot). The reason why your guest OS doesn't is because it hasn't been partitioned.
You'll have to boot your Guest OS using a bootdisk that has parted or Gparted and repartition the disk to include the unallocated "sectors" of the space you just added.
HowToGeek has an article that goes over how to do this in much greater detail. The section "Enlarge the Virtual Machine's Partition" uses a Windows Guest but the steps to accomplish are done via a GParted boot disk.
In case you are using snapshot, you need to get rid of those if you enlarged the base VDI.
But be warned it is not supported to enlarge snapshots and can actually corrupt your disks, especially when doing it on the base disk.
Be sure to make a backup of your VM if you care for it.