When playing an amiga game using libretro you can enter retroarch, then select core options, and over there you can find screen size option...
OK, thanks, stupid me. Now I found the options to change this, but now there is the "funny" problem that when I change the resolution, nothing seems to work right... The best result is achieved using "your" resolution (640x256) - but the emulator then only covers half the screen...
About savestates, as I said this core is totally experimental and its creator did not make any improve since its firts version, it would be nice to add savestates features.
Well, then let's pray (or beg him)...
Yours,
tu