The total size of all Nintendo 64 ROMs depends on the number of games and their sizes, but on average, an N64 game is about 8–64 MB. If you consider all released games (around 393), it would be roughly 10–15 GB in total.
All Nintendo 64 ROMs together take up around 10 to 15 GB in total, depending on the region and versions included. Each game is usually between 8 MB to 64 MB, so the full collection isn’t very large by today’s standards.