Use circular knitting needles. The longest I've seen is 40" (not the needles, but the plastic tubing in between). You should be able to do a good size afghan on those. It doesn't have to be 40" or under, it can be bigger. According to my understanding, you should be able to do something up to 3 times the size of the needle (I've never tried, so I don't know for sure.)
You'd want to use circular needles, and work back and forth as you would on regular needles. With 47in. circs, I see no problems making a good-sized blanket, other then how heavy it would get on the needles.