It wouldn't be that hard. Emissions standards can't force large footprints on electric cars, which we also need to transition to.
1) Stop suppressing gas prices. Don't allow the national oil reserve to be burnt up in private cars and people will stop buying large vehicles. The perennial oil crises show us this. This may happen inevitably because the oil reserve is draining at an alarming rate.
2) Remove tariffs on EVs to make domestic companies compete.
3) Subsidize small EVs built domesticly. EVs in general were subsidized until recently, so it's not unrealistic.
4) If you want a truck for truck things or the aesthetic, you can have a normal sized truck.
I'm not conceptually against Kei trucks, especially in cities, but a truck that is disallowed from interstates will find few US buyers.
The wiki article section on US legal status mentions the most liberal states on the issue still not permitting interstate operation.