Finger width or length? From the airlock, or from where the curve of the carboy/jug starts narrowing? I love details, as much as I can get!
If it's still fermenting then yes you should be safe with a little headspace, assuming you aren't constantly opening it to take readings. But the next time you rack (which will probably be pretty soon, you should rack when fermentation finishes, I sometimes wait a day or so extra so that I can make sure the most sediment possible is being left behind when I rack) you are going to have to deal with that headspace.
There are lots of threads here on dealing with headspace - the main way to deal with it (which you can't do now) is just to ferment in a bucket, or larger carboy, and make a little extra than you'll need, so that when you rack it will fill it all the way up. That only saves you from headspace for your first racking though.
I bought a huge pile of fish-safe glass beads (fish safe is important, means they won't leach poison into the mead) and I use some of those to take up the extra space every time I rack. They're a pain to deal with, but they've served me well.