Depends on the rom, could trim a 128MB rom down to 68MB if the developers went just over the 64 mark, or could trim a 128MB rom down to 126 if they filled the whole thing up. It really varies, there is no average or anything.
I trimmed Mario Kart on three of my kids DS's and the mulitplayer refused to recognize the other players for a multiplayer game. After I changed all the Roms to their full versions (untrimmed), the multiplayer worked and recognized the other players. I have yet to try it with other roms.