The biggest DS games are 120megs right now, that would be for a game like Jam Sessions because it uses soo many sound samples to create it's guitar sounds. A game like Mario64

S is only 8 megs on the other hand.
One thing to keep in mind is that these Games are measured in Bits, not Bytes. It takes 8 bits to make just 1 byte, so a game thats 128megabits is only really 16megabytes.
I used to work on the fringes of the game industry and still have friends that do, so I take posts like this seriously.