Not entirely true. The top selling US cars are way over 250,000. More accuarately is the the top selling US PREMIUM CARS are 250,000. Thus the debate over what is "mass market".
Here is my source: Top of the Charts: The Best-Selling Vehicles of 2011 - WOT on Motor Trend
The top selling cars in the US are mid-sized sedans, Camry, Altima, Fusion, and Sonata. The highest selling compact is the Corolla, with the Cruze and Civic close behind. Going by those top twelve vehicles the Mid-sized sedans added up to 1,288,000 and compacts were 693,000. That's 2:1 popularity of mid-sized sedans to compacts in the top selling cars in the US. Going by those numbers I'd think that Tesla should make the Bluestar a mid-sized sedan. Tesla is a worldwide brand - so that should definitely alter the overall equation. But it should be easy to keep their luxury status with a mid-sized sedan. I think it's trickier to convince someone a compact car is a luxury car.