Max size at launch is 2GB, likely will expand to 4GB in a year or two afterwards, and 8GB near the end of the system's life.
Common size is another question entirely. Looking back at the old DS, the max capacity for DS games at launch was 128MB (later expanding to 256MB and 512MB). The issue was that most DS games at first didn't even use nearly that much (still don't). Mario 64 DS was on a 16MB DS cart, Megaman Starforce was on 32MB, Mario Kart DS on 32MB. 32MB was quite a common size. But even today, there are still games released on 16MB cards, some even smaller. Giana Sisters DS was on a measly 8MB. Polarium was apparently below 4MB when trimmed...
You can expect to see some games that use 2GB. Likely ones such as Resident Evil Revelations, Metal Gear Solid 3D, and Kingdom Hearts 3D. Yet others won't use nearly as much. I wouldn't be surprised if some of the games are on sub-100MB game cards...