Since I decided to use Linux, because I like it a lot, I knew that I'll need Windows anyway due to the fact that everyone (at most) uses it and considering also that my posts would have to be focused in this operating system (my readers use it).
To overcome this situation I decided to install a virtual machine on my Linux (Debian specifically) to avoid rebooting anytime I need to do something in Windows, to accomplish this purpose I installed VirtualBox and installed in it Windows 7.
However, In some point there was need to get more disk space but I couldn't get it because when you define a virtual machine you specify a determined disk size. But, it can be done the following to increase the disk size using the tools provided by VirtualBox.
First, increase the disk size by using a VirtualBox command created for this task. Remember that the virtual machine has to be turned off to be able to modify the virtual disk, once you are sure about this, go to the configuration of the machine.
Then, go to "Storage" section and pick your virtual disk, this is just to get the physical path of it in your computer, copy it from "Location" label.
With this path copied go to a command line and execute "vboxmanage modifyhd <your_disk_path> --resize <new_size_in_megabyter>", it's very similar in Windows so you won't have any problem, in this example I'm increasing the size to 26 GB.
Now you have your virtual disk with the desired size, but as a physical disk it's necessary to asing the new space to the operating system, to do this boot the virtual machine (in my case Windows) and go to "Start", right click over "Computer" and click on "Manage".
In the new window go to "Disk Management" and you will see not asigned space (consequence of the size increment), just right click over the partition next to this free space and select "Extend Volume...".
Finally, select the amount of bytes you will need in the new partition and that's all, more space for your virtual machine.