In theory, not a very difficult thing to implement. The RAM size register is mainly used to restrict the accessible main RAM to 4MB before launching a DS game. I felt like looking at another of the known DSi-mode issues: the fact that we currently don't implement the RAM size register in SCFG_EXT9. Sometimes I wish all issues were this simple. The issue was another unimplemented AES feature, and was fixed in melonDS 0.9.3.
One example of a fun issue that had been reported a while ago: the DSi menu would freeze after the health/safety screen if any pictures were stored that could be displayed on the top screen. However, the road to DSi emulation is paved with all sorts of challenges. Not just in the sense of running DSi games faithfully: a good reproduction of the DSi environment is also useful to would-be homebrewers. You might have noticed that one of my goals for the 1.0 release is to get DSi mode in melonDS up to par with DS mode. If you're running into trouble: Howto/FAQ While it is still a work in progress, it has a pretty solid set of features: MelonDS aims at providing fast and accurate Nintendo DS emulation.
