Strange question
there's no the NORMAL size for the game: the smallest game I see is Namco Museum (9Mb in .zip) to 780Mb of Skies of Arcadia. Think about multi-disc games
So, I think you want to know what the normal size for the current game you want to d/l and you suspect that it's not full or very down-sampled? If so, then goes to www.isonews.com open up releases->dreamcast and read there .nfo files where you can find the actual size of the rip.