Just remembered to respond to this. Unfortunately, I'm not sure if there's a way to change the fullscreen resolution with RMXP games. Maybe there is and I just haven't seen it (Since I never used fullscreen in general while playtesting since it just gets a bit blurry), idk. I've tried looking it up but the closest I've seen is a controls setting (Which isn't very close at all).
I guess it makes sense, as XP was originally released in 2005 (2004 in Japan), but still.