Not that large, not even close. The TV business has changed enormously since the days when TNG got 20M viewers. Only the very biggest shows - American Idol, Dancing with the Stars, football - ever get over 20M anymore. 2010-11 Season-to-Date Ratings.I recently confirmed Star Trek The Next Generation did in fact over around 20 million viewers per episode and it is on the Wikipedia page. If I a new Star Trek show was to go into production shortly after the next movie, we can expect large viewership.
In TNG's day, 20M was what a niche show got. Now it's what only a handful of top shows get. Niche shows get 4-5M and then they get cancelled unless they're on the CW or cable.
Here's a model Star Trek might be able to make use of: Falling Skies was a solid hit on TNT with 6M viewers. Of course that's with Spielberg's name and the promise of a lot of alien-killing mayhem, but that does show that the market for space-oriented sci fi has not vanished, and can pull decent numbers on cable under optimal conditions.
I'm not holding my breath waiting for either of them to change their strategy. Just look at their lineups for the coming season: more cop shows and mass-market sitcoms for CBS; more young-female-oriented shows for the CW. If they were thinking of expanding beyond that, we'd be seeing at least a glimmer of it now. CW's boldest move was to pick up Ringer because it's aiming at a slightly older female audience than usual.The comments about CBS and CW having absolutely not being an outlet for a new show is bogus.