it seems to me quite obvious how the number got picked and why it became a hard limit: it would be very difficult to sell a bill that's any bigger. A health care bill much bigger could be plausibly rounded up to a trillion dollars by the opposition, and though the American public is still somewhat blinded by sticker shock from the last eight years of deficits, $1 trillion still sounds like a lot of money.
Has McArdle been following the health care debate? Does she not realize that describing the under-$900 billion Senate bill as a "trillion dollar bill" is a routine Republican talking point? Even if this wasn't already happening, doesn't she remember that the exact same thing happened with the stimulus? There may be good reasons for $900 billion but that's not one of them.