Those limitations have since been revealed. According to an FAQ posted on Bethesda’s website (via GameSpot), it has been revealed that mods on the PS4 will only be allowed to take up a maximum of 1GB of storage, while the Xbox One will have a bit more leeway at 5GB. “For Skyrim Special Edition, Mods will reserve 5 GB and 1 GB of space on Xbox One and PlayStation 4, respectively.”
This might be a bit limiting for some players, in which case we guess you’ll have no choice but to more selective with the mods that you choose to use. Also note that as per Bethesda and Sony’s agreement, mods that require users to upload their own assets will not be allowed, so basically mod creators will just have to work with what they’re given.
Like we said, it does feel a bit limiting but at least the option is there should players want to use mods. That being said considering how beautiful the game looks, perhaps this is a small price to pay.