Most emulators (Citra, Panda3DS, Lime3DS) cannot run encrypted ROMs directly. Therefore, many users prefer a "decrypted" or "CCI" set. Additionally, "trimming" removes dummy data (empty padding used to fill cartridges). A trimmed ROM is smaller but functionally identical.
The size of your set is heavily dependent on the you choose.
A full "No-Intro" style set containing every title released across all regions (Japan, North America, Europe) is approximately 1.7 TB to 1.83 TB North American (NA) Set:
