Well, I havent exactly tried it, and I really should before i post this, but I believe that putting a dollar sign ($) at the beginning of the filename will fix this problem, and the program will accept it, no matter what the filesize is, based on making the image fill up the entire screen.
Furthermore, I am not 100% sure this is right, because I only have tried it in RMVX...
EDIT: I just tried to add a new parallax to my RM2k3, and it took a character sheet no problem... I didn't change anything... I just exported a charset from the game and imported it as a parallax...
Your problem is most likely not a size issue, but an improper filetype or color depth. Try readjusting your new panarama so that it is the correct color depth (change it to 256 colors), and the proper filetype (bmp, or png)