If I have to make a prediction, I bet that the stadium chosen will be the SoFi Stadium in LA, with FIFA accepting the size but keep 80,000 seats.
I am very skeptical on this. Word Cup is a high-prestige event, and I don't think that the pitch size will be outside the regulatory requirement. In that case, the participating teams will complain about it. I am wondering why the MLS stadiums are so small. Why the World Cup matches need to be played in stadiums where NLF is played? In this case, the SoFi Stadium is home to the NLF franchises Los Angeles Rams and Los Angeles Chargers. The only instance when this stadium hosted a (normal) football match was last year, when a friendly was organized between Los Angeles Galaxy and Chivas Guadalajara.
I don't know the specific conditions for NFL stadiums to be turned into soccer stadiums and whether any adjustments have to be made to the capacity, but in what sense do you think the stadium is small? If they can offer 80,000 seats, isn't that more than enough? Even if they had to reduce it to 70,000 for whatever reason, that is still a stadium size that is above average.
In the Premier League the biggest stadium they have is the Old Trafford with roughly 75,000 and then the numbers go down pretty quickly. That probably counts for most European countries. Spain has the Camp Nou, yes, and Santiago Bernabeu, but then it's the same thing. The numbers go down rapidly.