my racking hose, which fits bottle fillers, spigots and auto-siphons, isn't anywhere near the same size as the hole in my carboy bung, or carboy cap.
if you have a 3 piece airlock, you might be able to remove the float, and put the tubing over the airlock stem itself.
Otherwise, keep it sanitized, and when the krausen falls you'll be ok. do put something sanitary in the airlock...not just plain water.
vodka is popular, but so is a little bit of sanitizer solution, like Star-san.
if you primary in a carboy, you really need a 6.5gallon carboy, not just a 5gal (which is more suited to secondary aging). even then, some beers will blow off more than 1.5 gallosn of headspace (like belgian wit beers) so a blow off tube is a handy thing to have.
I choose to primary in ale pails to keep plenty of space, since they are usually 6.5-7gallons
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10