The command line tool
VBoxManage will let you resize your guest's hard drive. Open up a command prompt/terminal/whatever for your host OS, and change to the folder where VirtualBox is installed ("C:\Program Files\VirtualBox" on Windows hosts). From there, run
VBoxManage list hdds to see a list of available hard drives. Find the drive that you want to resize and copy either it's full path (after
Location in the output) or UUID to your clipboard for the next command. Next, run
VBoxManage modifyhd C:\Path\To\Drive.vdi --resize 30000. Replace C:\Path\To\Drive.vdi with either UUID or the full path to your drive image. The number after
--resize is the new size in megabytes. If you want to go larger than 30 GB, feel free.
Once that's complete, you still have to let your virtual machine know that the new space is available to use. For Win7, that's easiest to do from Disk Management.