You can, but it's pretty hair raising, but no worse that say shrinking your NTFS partition to make room for a real Ubuntu install on its own partition. You should back up any important data on both the wubi disk and the NTFS partition just in case. Also make quite certain that there is plenty of room on the NTFS partition for the changes you are about to make. Resizing the wubi partition to something larger (or even close to) than the available free space may stop Windows from booting, and the resulting crash might also break the wubi install.
The first thing you need to do is boot off the LiveCD or LiveDVD. This method uses Linux tools, and you can't do it from within Ubuntu on the virtual disk you are resizing. Select "try Ubuntu" when prompted. Wait for the desktop to load up.
Next, open a terminal window with ctrlaltt
We need to find out which partition Windows is on so that we can mount it at a known location and resize the wubi disk. In the terminal windows type
sudo fdisk -l
You will be looking for the NTFS partition of the right size. Most likely it will be /dev/sda1 or /dev/sda2 if there is a recovery partition or a windows bootloader on the drive. Do not proceed with these instructions if you are not sure, ask for further help by posting the results of the fdisk command into your question.
Next, we will mount the NTFS partition to get access to the wubi drive. In my examples I will use /dev/sdaX but you should changed this to the correct drive as determined in the previous step.
We are not ready to mount the NTFS partition:
sudo mount /dev/sdaX /mnt
Next, run a check on the filesystem on the wubi drive:
fsck -f /mnt/ubuntu/disks/root.disk
If you get errors do not proceed until they are all resolved and the fsck gives a clean result.
Lastly, resize the wubi virtual drive!
resize2fs /mnt/ubuntu/disks/root.disk 30G
which will resize the virtual disk to 30Gb