I usually set a big amount of space for my VMs, but this time I created an Ubuntu VM with just 15GB of dynamic allocated space. I wanted to use it "just to test stuff" and I knew I would probably make a mess and that VM won´t last much. But, all things went more than great and I ended up using it as my "prod" environment for now, this is were the guest started to complain about the limited space left, I did a quick search in Google and I got the following:

1. Shutdown your VM
2. Make a copy of you VDI, you can just ctrl+c, ctrl+v
3. VirtualBox comes with an utility called VBoxManage.exe which is usually located at "c:\Program Files\Oracle\VirtualBox\VBoxManage.exe"
4. To modify the size of your VDI to 25GB execute the following command:

C:\path\of\your\vdifile\"c:\Program Files\Oracle\VirtualBox\VBoxManage.exe" modifyhd yourvdifilename.vdi --resize 25000

Output:

0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

Start your guest VM and in my case it registered the new space automatically but if your guest didn´t you can follow the instructions detailed here: http://derekmolloy.ie/resize-a-virtualbox-disk/

Props to derekmolloy from where I took this information