I'de say it was rather large, since looking around, it's believed that it was very big pre-WWiII, but a quick look for numbers isn't revealing any good sources. I suppose it'll just have to float around as a "general belief" that the Japanese automotive industry was "rather big" pre-WWII.

WWII definitely effected the US less than a lot of countries though that is for sure. But I think certain industries benefited more from USA's post-war advantage than others (automotive maybe being an example).