The size given is the size of the coma, not the core. The coma is the gas given off as it approaches the sun. Remember Comet Holmes? Its coma expanded until it WAS the size of the sun. The core of Comet Elenin is at most 7 miles wide.
Now, that doesn't mean there might not be some interaction between the sun - the coma of Elenin and the earth, but that doesn't mean we're talking about a huge object here!
Please take all the stuff you find on the Internet with a grain of salt. There may very well be highly interesting things happening with this comet - -possibly even fulfillment of prophecy -- BUT, this particular object is not a brown dwarf or huge in size. Some Planet X may still be out there (and some people think that if it is, its approach might be very, very fast at the end), but Comet Elenin is a comet, and a fairly small comet at that.
Now, with a coma that large (and probably even larger as it gets even closer to the sun), we may actually be able to see it in the sky (although some are now saying it will be a Southern Hemisphere event), and that could be rather cool. And, it may yet fulfill prophecy. But, the only thing that's huge is its coma - and it's not unusually large for a comet's coma.