Resize VirtualBox Disk Image – grow, expand, increase a .VDI disk image in 1 minute

 Firs link should solve your problem, in cose it does not,  try others :)

WebDesignBlog 

Increase the size of a .vdi file

http://www.my-guides.net/en/content/view/122/26/

Resize VirtualBox Disk Image vdi files to bigger size

https://hezral.wordpress.com/2008/10/18/resize-virtualbox-vdi-files-to-bigger-size/

How to Resize a VirtualBox vdi file

http://thestandardoutput.com/2011/05/how-to-resize-a-virtualbox-vdi-file/

Expand vdi virtualbox disk image

http://blog.markloiseau.com/2010/05/expand-vdi-virtualbox/

Grow a virtual drive, vdi Virtual box disk image

https://forums.virtualbox.org/viewtopic.php?f=3&t=17512
Source of the easy way:
http://www.webupd8.org/2011/02/how-to-easily-resize-virtualbox-40-hard.html