Hi, current Grafx2 coder here. I tested the loading of IFF images from many sources, but about the saving, I only tested compatibility with Deluxe Paint 2E, the MS-DOS version which runs in 256 colors too.
Grafx2 always saves a 8-bit (256-color) IFF file, so this could be one reason for Deluxe Paint 3 or 4 to choke on it (if you use colors above 64, or your 64-color palette isn't EHB).
From the comment by Akira, I also checked why screen size matters :
Grafx2 saves the screen dimensions in the IFF header. It's asked by the format specification, and in MS-DOS times, it allowed Grafx2 to automatically re-open an image in the resolution it was designed.(even crazy modes for a PC, like 320x512)
If Deluxe Paint 3 or 4 tries to make a canvas of this size, it's him who's not following the written standard :/ But it won't be the first time I'll adjust the program according to "what works".
Can you give more details about what happens on re-loading an image in Deluxe Paint 3 or 4 ? and can you try to reduce the Grafx2 window to its smallest before saving (before you open the "Save" dialog), it will be exacly 320x200, a resolution that DP should know.